Do you have app passwords set up and in use in your k9 in/out-bound mail configurations? Gmail wants clients to use OAuth2 but as K9 doesn’t support that yet you need to go the app password/less secure app route. See: https://forum.k9mail.app/t/gmail-setup-could-not-finish/650 (e.g., the link in my first response in that item) for more details.