You cannot assign materials to specification type &

What causes this issue?

Each specification is assigned to a specification type, such as real
substance or listed substance. In Customizing, you specify for each
specification type whether a material can be assigned to this
specification type.
The specification you selected for assignment belongs to a
specification type that does not allow material assignment.

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?

Error message extract from SAP system. Copyright SAP SE.