Mine is solved now.

So what I guess is going on is the following:
If, before you've completed the color challenge, have done ANYTHING to the sets that were meant for you to participate in the color challenge (these would be Keep the Peace, Aerial Domination etc...) --> then you get this problem.

I contacted customer support. They reinstated the two sets that I had had problems with, and every thing worked fine.

I FIGURED THIS OUT!

This worked for me but this may be a rare case.

I was getting the “Network Error. Unable to join event. Please try again later.” message for the fifth and final step of the Red Color Challenge only.

The fact that it was only the Red quest told me it had something to do with the Red Deck: Goblins Everywhere!

Then I remembered I had opened some packs and received a Mythic Rare card that I obviously immediately added to the Goblins Everywhere! Deck. Apparently that is a big No No and stops you from completing the color challenge because the fifth step is treated as a Standard game I believe.

So all I did was remove the Mythic Rare from the Red Deck and badabing badaboom I completed it.

This took me two whole days of troubleshooting, and WOTC needs to fix the error screen to properly explain what’s going on. Yikes this company is whack.

why dont they just remove the last challenge on all colors for what I see this is only is happening on the last challenge I m stuck with the last red challenge the is all done

I'm getting this error during the very first White challenge where it is White vs ?

So... they just keep ignoring this? No response AT ALL?

I've completed all the color challenges but I don't have the pre-built decks on my account. And when I try to go to the color challenge for any of the colors, it tells me I've completed them. Then when I try to play one of the color challenges I get a network error.

Can’t complete color challenge. I keep getting network error for trying to complete last challenge which is the pvp.

I see a lot of people are having similar issues and normally I wouldn't care but thanks to the new UI this "challenge" is at the top of list all the time. Let me finish this thing and/or get rid of it somehow.

Same issue here. Any other ways to get the reward and unlock the other modes?

I cannot complete the final battle of the color challenge to unlock pvp. The last battle of each colored Deck just gives me the attached error notice. It also looks like none of the cards I have unlocked are in my decks.

Network Issue for White and Green Color challenge ..

Have tried restarting app- did not resolve
Tried delete/reinstall app- did not resolve

Gives this "network error" on an offline match (the ones to master the colours, I hope I'm explaining myself

I can't complete color challenge as reported by other players. After completed challenges against other colors, it's impossible fight against white and obtain rewards because the system gives the message "network error: try later" . So, when do you fix the bug?

I have not been able to challenge the fifth person in any of the colour challenge games. It's saying "Network Error". Please help?

Trying to complete the color challenges and it gives me a network error on the 5th one of each color. It happens on PC.

I'm having this issue to, can't fix it :/

After submitting a support ticket, they gave me all missing decks and was able to do all my 5 final steps of all color challenges !

I followed each steps provided by @valent0ne#05451 below.

The I can't complete the color challenge because challenge V on White just gives me "Network Error - Unable to join event. Please try again later". Other people have had this problem too since November from what I've read. When's the fix coming?

Has the dev team posted ANY acknolegment of the issue? This is a terrible introduction to the game.I have the same issue.

Why is this not listed under "Known Issues" - it's been ongoing for a year.