Character &1 is not valid. Valid characters are & and &

What causes this issue?

You have used a character (&V1&) for the coding of the project
definition or the WBS element that the system does not recognize.

System Response

INCLUDE 'NA_ALLG_NOVERARBEITUNG' OBJECT DOKU ID TX

How to fix this error?

Check to make sure that you have entered the correct character. Change
it if necessary.

Procedure for System Administrators

Check whether the character you want to enter for coding the project
definition or WBS element has been defined as such in the system.
Define it, if necessary.

Error message extract from SAP system. Copyright SAP SE.