Saving not possible; field & with filter & has not been assigned yet

What causes this issue?

You use the <ZK>quantity</> selection criterion in the filter type.
You have not, however, specified the <ZK>base unit of measure</>
field.
You must specify the base unit of measure so that the quantities can be
converted into the corresponding base units of measure of the selected
products.
The units of measure used must be able to be converted for the
individual base units of measure. You specify the conversion in the
product master.

System Response

Saving is not possible.

How to fix this error?

Assign the <ZK>base unit of measure</> selection criterion
to the filter type.

Procedure for System Administrators

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.