Number & not defined for material type &

What causes this issue?

Every material type has a speific number range for external number
assignment. The material number you have entered does not lie within
the number range for this material type.

How to fix this error?

Choose a material number that lies within the number range of your
material type.

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

Error message extract from SAP system. Copyright SAP SE.