Hi Sam,
Most likely the SAP* user is not connected with a PERNR in IT105 
That's why the FM will not find a reference point to give access to part of the OM structure.
May I ask why you'f want to assign a restricted structural profile to SAP* instead of actual users? Are you trying to assign a structural profile to all users by restricing SAP*'s profile?
Good luck!
Dimitri