Found a solution on this link. It worked for me, don't know if it will for other people.
Accepting a contributor invitation – Squarespace Help Center
Error message: You already have permissions on this website
This message appears when try to accept a contributor invitation while already logged in with an account that has permissions on the website.
If you are trying to add permissions on a different account, we recommend accepting the contributor invitation in Incognito mode. That way, you can log into the correct account after clicking Accept Invitation.