Hast das mal probiert https://www.reddit.c..._challenge_bug/ ?
I, like others, have been unable to complete to color challenge. Wizards has recognized the issue and provided instruction on how to skip the challenge here: https://twitter.com/...2672222211?s=21
Once you skip the color challenge, you will have access to battle other players. This option will allow you to work around the bug.
Step zero: Ensure you have unlocked all game modes as mentioned above. This can be accomplished via Options > Account > Skip tutorial.
Step one: Click the play button in the bottom right. This will bring up a tab on the right side.
Step two: Under “Find Match” select play.
Step three: Click play at the bottom right.
Step four: Immediately click cancel before it finds an opponent.
Step five: Click play at the bottom right again, bringing up the right side tab.
Step six: Select color challenge in the tab.
Step seven: Click play at the bottom right. This will navigate you to the color challenge.
You should now be able to get a match and complete the 5th step of the challenge. You will need to repeat this process for each color. Also, after step 4, I was still able to complete a challenge by clicking the color challenge bubble and starting the match through that route.