MRP area for &1, &2 not determined - continue with plant MRP aea &3

What causes this issue?

An error occurred while defining the MRP area data for material &1&,
MRP area &2&.

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.