Multi-set & is not allowed

What causes this issue?

You have tried to compare a field value with a set which is assigned to
a table which is not assigned to a Boolean class. This set, &V1&, is a
multi set and is therefore may not be used in connection with a field
value.
<ZH>Example</>
,, <BSEG> $HKONT in <CCSS> MULTI_SET

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

System Response

The entry was refused by the system.

How to fix this error?

Try to use a basic set or a single set. With these types of sets, it
is possible to make comparisons with field values.
<ZH>Example</>
,, <BSEG> $HKONT in <CCSS> SINGLE_SET or
,, <BSEG> $HKONT in <CCSS> BASIC_SET

Error message extract from SAP system. Copyright SAP SE.