In one section of Project or WBS Element, there is no number. "&"

What causes this issue?

You have maintained characters in the key for the project definition or
WBS element, that are not valid according to the coding.

System Response

INCLUDE 'NA_ALLG_NOVERARBEITUNG' OBJECT DOKU ID TX

How to fix this error?

Enter the key as defined in the coding.

Procedure for System Administrators

Check if you have properly maintained the coding for the project
definition or WBS element in Customizing. Make any changes necessary.

Error message extract from SAP system. Copyright SAP SE.