You have not assigned an MRP group to the material

What causes this issue?

You have not entered an MRP group in the material master record.

System Response

If you do not enter an MRP group, the material type takes on the
function of the MRP group. However, this will only happen if an MRP
group has been set up with the material type key in Customizing.

How to fix this error?

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.