Only letters are allowed at the present cursor position

What causes this issue?

When entering the number of the functional location, you have used a
character at the current cursor position (&V1&) that is not a letter.
The character is not allowed in its current position.

System Response

INCLUDE 'KEINE-VERARBEITUNG' OBJECT DOKU ID TX LANGUAGE EN

How to fix this error?

INCLUDE 'AUFBAU' OBJECT DOKU ID TX LANGUAGE EN

Error message extract from SAP system. Copyright SAP SE.