In order for a tag (or any other type of credential) to be able to open a lock, all access rules have to be defined correctly.
Please check whether:
The lock is part of an access group.
The lock is online (this is visible on the locks overview page)
The user is part of the access group.
The time schedule as specified in the access group is valid at the moment of testing.
Please reach out to Salto support if following these steps did not solve your issue.