User &1 is not a reference user (see long text)

What causes this issue?

You enter user &V1& as reference user. However, this user does not have
the user type "reference user".

System Response

The system displays a message.

How to fix this error?

You can set the message type of this message using a Customizing switch.
To do this, make an entry in the table <ZH>PRGN_CUST</> containing the
ID <ZH>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

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.