SAP Message 01391 - User &1 is not a reference user (see long text)

Diagnosis
You enter user &V1& as reference user. However, this user does not havethe user type "reference user".

System Response
The system displays a message.

Procedure
You can set the message type of this message using a Customizing switch.To do this, make an entry in the table PRGN_CUST containing theID REF_USER_CHECK and one of the following values:
W - Warning (default, no entry required)
E - Error message. Users that do not have the user type "Reference"cannot be entered as reference users.
S - Message
I - Ignore user type, no message