No condition type group for purchasing is assigned to the promotion

What causes this issue?

You want to create purchase price conditions.
However, no condition type group for purchasing is assigned to the
promotion.
The condition type group that is assigned to the promotion defines
which conditions can be created for the promotion. The condition type
groups are defined in Customizing for the promotion.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Enter a condition type group for purchasing in the header data of the
promotion.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.