Hello,
by default the username is case insensitive, the password is case sensitive.
After you have tried all possible combinations lower or upper case first letter, if this does not work you may need to perform a password recovery procedure.
You need a console connection. You need to start the console connection client before powering up the switch.
Follow this thread
https://community.cisco.com/t5/switching/catalyst-9300-password-recovery/td-p/3433758
They speak about catalyst 9300 it should apply to your device too.
Hope to help
Giuseppe
