I tried Hilapdtaus's solution and it didn't work. I'm glad it worked for him though.

When I tried that approach it asks me to save changes to the deck, then after clicking "ok" a network error occurs saying I am "Unable to update deck. Please try again later."

I figured out what worked for me, maybe it'll work for you too!

Like others had said, when I went to edit my deck for the final challenge, all of the cards were shown as red meaning that they were ineligible for play. The deck format was set to Standard and I couldn't change that.

I removed all of those cards. This left me with land only. I then added what cards I had that were legal for play. I stuck with just the color of the challenge that I was doing -- I do not know if that was necessary, maybe I could have used a multi-color deck? Every opponent I played against was using multi-color decks...

Anyway, I stuck with just the color of the challenge that I was completing. I didn't have enough cards to make a full deck so I just added what I had and filled in the rest with land. A lot of land.

I then played the challenge. I got spanked in every single match. However, each challenge was marked as complete and I received the reward cards for completing each color challenge!

PvP is unlocked and I received a whole bunch of wild cards and some other stuff.

It seems as if Wizards does not really even look at these posts at all. Its not super breaking at this point, but it is really annoying that this even exists. And the countless bug reports submitted to try and fix this, going un-noticed.

I've tried every solution on here and reddit but nothing has worked so far - i even bought the 2021 and 2022 arena starter kits to hopefully give me enough cards to build a valid deck since the default one for challenge 5 for each colour has the decks showing invalid cards (i think it's because i played briefly a long time ago and only just came back).

When i went to my actual decks tab it said that the lands for each deck were invalid?

I've tried reinstalling the game, building a totally new deck of appropriate coloured cards (get the 'can't update deck at this time' error), starting the tutorial from scratch, starting and then skipping the tutorial, playing on android and PC, crafting missing cards (though apart from an hour stint a year ago i'm a new player so don't have the amount of wildcards needed anyway)

Is there a solution available that i've missed?

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Completed all other challenges but can’t complete last black colour challenge. Whenever I try to start the match it says “unable to join event. Please try again later”. I need to be able to do this last challenge to unlock PvP. Playing on IPhone 12 IOS.

I fixed it, I added redeemable codes to my account before actually playing the game - this made it so the starter deck had illegal cards or something. This made me unable to join a game and I got this annoying and extremely vague error. Anyway I removed all the cards in the starter deck of the color I was trying to play with and added random cards until it was valid. I found a match, conceded immediately and now I can actually play the game. Hope this helps.

OK, so this problem has been going on for over a year and there is still no fix? How is this even possible?

Seems like a great game if only you could get past the tutorial "Color Challenges" - I get the same network error on the last match as everyone that gets. Trying to play on Google Pixel 5. Super disappointed to read this thread

I tried it on my own account and get this error too.

There seems to be a mismatch between the deck that is used in a Final Color Challenge and the deck that is expected. I notice that the decks that cause a Network Error have missing cards. Some have less than 60 cards, and some have over 60. I also notice that the decks in the Final Color Challenge are set to Standard when in fact they are only valid in Alchemy format.

If you compare the deck in the Final Color Challenges with the same one in the Starter Decks folder, you will likely have a mismatch in terms of decklist. I think this mismatch is a likely cause of this bug.

Hopefully, some devs can use this information and fix this bug that is likely turning new players away.

because I have cards from new capenna that match the UNEDITABLE color deck, black has 62/60 cards and I cannot play or complete the challenge

I can’t do the last color challenge to finish the quest and unlock pvp.

Having this problem too; Wizards not interested in new players?

Couldn’t access last challenge of the last color, it says “network error”. As suggested in the comment, I opened the deck from the deck builder but I couldn’t solve the issue, I opened in the color challenge and I could see the card that is supposedly missing and the wild card icon but I couldn’t craft nor solve the issue in any way. THIS IS EMBARRASSING for WotC.
Fortunately I solved the issue (for IPad user but should work for other device as well):
1. Close the Magic Arena app (not minimize but properly close it)
2. Click on the Magic Arena App until starts to flicker and shows a small “minus” on the top right corner
3. Press the “minus” symbol deleting the app
4. Go to Apple store
5. Write in the search bar “Hearthstone” and download the corresponding app
6. Enjoy yourself

I managed to convince a friend to try the game and now he is faced with this error that after some research I see that it has lasted for 2 years, how is it possible that WOTC leaves a bug that influences the experience of new players to last 2 years???

Unable to complete color challenges. The last one in each gives network error. When I open the decks it says there are illegal cards but doesn't show what ones are illegal.

I am having this issue as well as a returning player. I have tried all of the suggested fixes (redoing tutorial, queuing into a normal game and cancelling) but nothing has worked

Having the same issue Cannot complete the last colour challenge with Lilliana - saying "Network Error". I can not complete the tutorial. Very frustrating!

Also having this issue with the last phase of every color challenge. Just gives "network error" message. Stopping me from progressing through the tutorial.

Unable to complete color challenge. Get network error every time on the last step. Looks like a well experienced issue.... Guess it's not getting fixed.