Log into switch account on steam?

I played this game first on the Switch. Is there any way I can play on steam and log in to the existing account I made on switch? Is there any form of cross progression?

Yup! Switch and Steam are synced with the same accounts in-game.

Then I seem to be having a problem because in the steam version, before letting me online, it is only giving me the option to create an account. It isn’t letting me log in to an existing account. And when I try to put in the email address of the account I used on switch, it just says that the email address has been taken. So how do I link steam and switch?