Specified material already assigned to material type &

What causes this issue?

You have selected the function "Create special". However,
the specified material has already been assigned to a material type
that is not the material type defined for this function.

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?

Either select the function "Create general"
or select the function "Create special" which corresponds
to the material type to which the material has already been assigned.

Error message extract from SAP system. Copyright SAP SE.