When adding a new user mapping, your receive an error message ""There was an error! Your changes were not saved."
Search the CRM to see if the user have possibly two user records in the CRM, try to search the CRM ID in Outreach by going to the CRM>User object>Record Mappings, then type the ID in the search box. You cannot search the record mappings by the user name, this field only search by using the Salesforce ID. If you find an old mapping, you can delete it by navigating to the ... menu next to the record mapping, then select Remove. Once removed, you can add the correct user mapping by following the steps in this article.
This usually happens if the user is already mapped in Outreach, most likely with a locked user.