I want to change my handle to Mina but when trying to do so CF says "This handle is currently in use" although it has been inactive for 7 years and didn’t submit any submission.

How does this handle request thing work? Should I ping Mike?

Testdata doesn't satisfy the constraints mentioned in the problem statement, you can check yourself

Here's the code I used to test this:

https://codeforces.com/contest/1771/submission/184796886