I've dealt with the Windows 10 Mail App before, but have not encountered this issue. When trying to set up an account, which is a Google account, but for another institution, that is, it doesn't use @gmail.com, but is for @school.edu but is managed by Google.
I got through the account creation and granting the App permission to manipulate e-mail, calendar, etc., then the app closed.
I thought, OK, let me open it again, but I'm back to square one on the create accounts page and if I try to create it again I get a message it already exists, however the, "Ready to go" button remains stippled out, forbidding further progress.
Any insights would be appreciated.