1. Look for Report a Problem near the top right of any Support Community page. Click on it and write up the problem you are having. This should get you some individual attention on how to fix the problem. Three things to put in your initial report:
a. If you have already tried uninstalling and then reinstalling the app, and it didn't fix the problem, let them know that you already tried this and it didn't work. This is usually the first thing they tell mobile app users to do when they have a problem.
b. If you can, enclose a screen print showing this message. They have almost certainly gotten reports of this issue before, so getting a screen print now is not going to give them anything new. But if you don't enclose one at first, they will write back and ask you to send one.
c. Remind them that you are using a Khan Academy mobile app and that clearing cache and cookies doesn't apply to your platform.
2. I suspect that this issue may be caused by timeouts. If you have a slow Internet connection, this may be why you are getting this message. This issue used to occur more often when Khan Academy was a slower site than it is now. I had this issue in mid-2018 when I stayed at a hotel with wifi that often filled to capacity and caused slow response times all over the Internet. Khan Academy really needs to adjust its timing out tolerance.