Quantity string & does not exist

What causes this issue?

The quantity string you entered does not exist in the system.

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?

INCLUDE NA_EINGABE_PRUEFEN
Table T156M lists existing quantity strings. Do you want to
<DS:TRAN.SM31>display</> this table?
Do you want to <DS:TRAN.OMBQ>check</> which quantity string is used for
a movement type by taking a look at the table of movement types (view
"Update control")?

Error message extract from SAP system. Copyright SAP SE.