Field &-& (data type & not allowed in matchcodes)

What causes this issue?

The field you are trying to include in the matchcode object has a data
type that is not permitted in matchcode fields.

System Response

As only data types allowing an associative search (i.e. those that can
be shown in CHAR format) may be used in matchcodes, the system will not
allow inclusion of this field in the matchcode object.

How to fix this error?

Use only fields of data types CHAR, NUMC, ACCP, CLNT, CUKY, DATS, DATE,
TIMS, LANG, UNIT in the matchcode object.

Error message extract from SAP system. Copyright SAP SE.