Enter a material in material group &

What causes this issue?

On the initial screen you entered 'material group" and materials from
different material groups.

System Response

The system only acceps materials from the same material group.

How to fix this error?

The material group is defined as that for the first material. In the
other items only enter materials in the same material group. If you
require materials to be entered from a different material group, create
another document for these materials.

Error message extract from SAP system. Copyright SAP SE.