Material &, unit &, layout module &, version &: Basic variant not allowed

What causes this issue?

You have not set the indicator for excluding the material in a variant
of the assignment of the material to a layout module. This is allowed
only in layout module variants if no basic variant for it exists.
As a basic variant does exist, the variant can be maintained only if
the exclusion indicator is set.

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.