Mask must begin with letter &

What causes this issue?

The special character that you used for coding the project definition
of the WBS element is not recognized by the system.

System Response

INCLUDE 'NA_ALLG_NOVERARBEITUNG' OBJECT DOKU ID TX

How to fix this error?

Check if you have entered the right special character. Change it, if
necessary.
INCLUDE 'NA_ALLG_VORGEHEN' OBJECT DOKU ID TX
Check if the special character that you want to use has been defined.

Error message extract from SAP system. Copyright SAP SE.