Even though the user that the script is running under has full Administrative access to the repository, it sounds like the user doesn't have permissions to THAT tag. With a security tag, trustees must be added to that tag in order to obtain permission to assign the tag or see any documents for which that tag has been assigned.

You can add that user to the tag's permissions by opening the "Properties" for that user within the Laserfiche Administration Console, then looking under the "Tags" tab:

You can view the users currently added to that tag's permissions by looking in the "Properties" for that tag within the LF Admin Console:

These trustees must be added on a tag-by-tag basis (note: this only applies to security tags, not informational tags). Also, Administrators don't get access to all security tags by default; that access is only controlled at the tag level, and must be added for the individual user.

After adding this user account to that tag's permission list, it should resolve your issue.