External number assignment not allowed for promotion type &

What causes this issue?

You entered a promotion number. However, external number assignment is
not possible for promotion type &V1&.

System Response

The system cannot create the promotion.

How to fix this error?

Omit the promotion number or assign an external number range in in
Customizing for <DS:SIMG.W_ZF_VK_0220>promotion type
</> &V1&.

Error message extract from SAP system. Copyright SAP SE.