The variant & (w/o exclusion) of layout mod. & for unit & will be deleted

What causes this issue?

You have assigned a material to a unit of measure of the basic version
of a layout module. However, there are already assignments to layout
module variants which do not exclude the material (exclusion indicator
not set). For consistency, the variants must be deleted because they
may exist only without the assignment to a basic version of a layout
module.

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?

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.