There is a possible solution found by r/WARKIDKAOTYK. I'll copy it here. This solution did not work for me, but it might work for you. Good luck.
----------------------------------------------------------------------------------------------------------------------------------
I encountered this myself. I have completed the initial five colors to unlock the five decks. It didn’t happen until I edited the decks that are available to use in color challenge from the decks tab. I had only edited two decks at that point and thought I was completely locked out. I was trying to use the black deck which I had edited pretty heavily but the blue deck was still useable as I only dropped two lands and added duplicates of two cards that were already present in the deck.
Spammed the play button a whole bunch of times then tried with the blue deck and it let me in. Didn’t think anything of it except the network error was resolved. Today I edited the white deck pretty heavily and it gave me the same network error then I tried the blue deck again. Got in. Tried the heavily edited black deck. Network error. Tried the unedited red deck. Got in. White deck. Network error. Black deck. Network error. Blue deck. Got in.
I think it has something to do with the card list for the deck you are trying to play with and will only accept decks with certain cards in it. It might only accept cards from certain sets. Idk yet as I am still trial and error on this myself.
I never had an issue until I messed with the deck builds tho. My suggestion would be to copy the deck you want to edit and edit the copy to use in standard play and save the original to use in color challenge.
I’m going to look up the original decks to try to restore to original and see how that goes and tell them from there to see what it will allow.
----------------------------------------------------------------------------------------------------------------------------------------
Edit: I fixed the problem. Put in a support ticket and ask them to re-add your color challenge decks. They responded in about an hour, and I was able to complete the challenge.
There is a possible solution found by r/WARKIDKAOTYK. I'll copy it here. This solution did not work for me, but it might work for you. Good luck.
----------------------------------------------------------------------------------------------------------------------------------
I encountered this myself. I have completed the initial five colors to unlock the five decks. It didn’t happen until I edited the decks that are available to use in color challenge from the decks tab. I had only edited two decks at that point and thought I was completely locked out. I was trying to use the black deck which I had edited pretty heavily but the blue deck was still useable as I only dropped two lands and added duplicates of two cards that were already present in the deck.
Spammed the play button a whole bunch of times then tried with the blue deck and it let me in. Didn’t think anything of it except the network error was resolved. Today I edited the white deck pretty heavily and it gave me the same network error then I tried the blue deck again. Got in. Tried the heavily edited black deck. Network error. Tried the unedited red deck. Got in. White deck. Network error. Black deck. Network error. Blue deck. Got in.
I think it has something to do with the card list for the deck you are trying to play with and will only accept decks with certain cards in it. It might only accept cards from certain sets. Idk yet as I am still trial and error on this myself.
I never had an issue until I messed with the deck builds tho. My suggestion would be to copy the deck you want to edit and edit the copy to use in standard play and save the original to use in color challenge.
I’m going to look up the original decks to try to restore to original and see how that goes and tell them from there to see what it will allow.
----------------------------------------------------------------------------------------------------------------------------------------
Edit: I fixed the problem. Put in a support ticket and ask them to re-add your color challenge decks. They responded in about an hour, and I was able to complete the challenge.