I can’t complete setup for StartMail account. It gives me an error reading that the username or password is wrong. But it’s not.
You are following this: https://support.startmail.com/hc/en-us/articles/4415515691409-K-9-Mail-Android-
Bonus points for a ISP that has K-9 instructions!
Just a guess, you did put the @startmail.com after your userid as seen in the fourth picture?