Steam is having serious issues so the backend is probably down.

Had to do this again today, still works for me at least.

Try what I wrote here: https://psteamcommunity.yuanyoumao.com/groups/idlemastery/discussions/0/490125103620240108/

Not gotten any drops less than 2 hours with six games.
I haven't been using Idle Master for a few days since it kept breaking on me (found a fix just now).
All games Steam-bought, I've asked for (and gotten) two refunds in the past.

I had the same problem, apparently caused by a broken configuration file.

Close Idle Master.

Search in the path C:\Users\<you>\AppData\Local\Apps\2.0\Data\{8randomchars.3chars}\{8chars.3chars}\idle..tion_86367c4a3014b007_0001.0004_0282557b99ab4040\Data\1.4.0.0 -- or anything like that.
The two sets of random chars are machine dependent, just look for something with a 1.4.0.0 folder and a file user.config in it.

DELETE THIS FILE.

Then log into Idle Master again. This time it should properly create the config file and you can idle all your games again.