Please enter one value only

What causes this issue?

You set the 'always on material group basis' indicator and also entered
a value-only material. In this case, the system is not able to decide
which of the two entries it should use.

System Response

If the 'always on material group basis' indicator is set, the system
uses the material group material of the current material group as the
default value.
If you entered a value-only material, this is entered after the check
for the selected material groups.

How to fix this error?

Deselect the indicator or delete the entry for the value-only material.

Error message extract from SAP system. Copyright SAP SE.