If you change Development Environment Authentication Schemes (aka the way you login to the APEX Builder) at the instance level and you don't read a confirm dialog carefully, as I didn't you may struggle to login as the instance admin next time:
In my case, I've changed it to the Database Accounts, and though that doesn't affect the instance admin account. But I was wrong. In my case, the solution was to create a database user with username admin.
After that, I've logged in as instance admin and changed the authentication back to the APEX Accounts.
If you change it to some other Authentication Scheme you may need to use PL/SQL API:
Link in the documentation.
That's all! Stay safe & enjoy!
Tested in APEX 20.2.0.00.20