MacOS Bug Sur can format drive but can't mount it
After a bit of &quot;googling&quot; I can see the problem occurs to other people too.
Try to follow the guide proposed below before you go a step further.
https://discussions.apple.com/thread/252126476
Just to reason a bit more on the problem: have you modified the settings of the stick via software or hardware ( e.g. moving an unlock button)?
This guys solved with a software different from Disk Utility, but I don't think it is necessary.
https://discussions.apple.com/thread/252081600