I think you are using Discourse’s implementation of SSO and are setting the require_activation SSO parameter to true in the SSO payload. If that is the case, have a look at this topic for details about fixing the issue: Debugging and fixing common SSO issues. It is safer to run the Rails console commands that are given in that topic than to directly update your database with SQL queries.
Merging the two accounts, as stated in the above post, is probably a good approach, but you may need to make some adjustments to the SSO record after having done that.