Blank unit field found

What causes this issue?

Your query includes currency amount fields or quantity fields. Each
currency amount field and quantity field has a reference field (a
currency key field or unit field) containing the relevant currency or
unit. On reading one of the currency amount or quantity fields, the
system discovered that the relevant unit field contained no value
(SPACE). This indicates that the reference fields have not been
properly maintained in the InfoSet used to create the query.
If the reference fields are not maintained, ABAP/4 Query is unable to
distinguish between different currencies or units and therefore does
not work correctly when calculating totals.

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?

Check the InfoSet. In the case of InfoSets which use logical databases,
this error should not occur because the database program has to ensure
that reference fields always contain the correct values.
With InfoSets which do not use logical databases, you may have to use
record processing code to ensure that the reference fields of the
selected currency amount and unit fields contain the correct values.

Error message extract from SAP system. Copyright SAP SE.