You cannot assign the activity number

What causes this issue?

The system could not assign a unique number to the activity while you
were creating it in a Gantt chart.

System Response

INCLUDE 'NA_ALLG_NOFUNKTION' OBJECT DOKU ID TX

How to fix this error?

Maintain the activities in time scheduling using the function
<ZH>Detail dates</>.

Error message extract from SAP system. Copyright SAP SE.