Currency type & is not allowed. The currency must be changed.

What causes this issue?

Currency type &V1& requires that currency in the controlling area must
be changed from &V2& to &V3&.

System Response

The SAP System checks whether transaction data exists in the
controlling area. The currency cannot be changed if transaction data
does exist, since this would lead to data inconsistencies.

How to fix this error?

If transaction data exists, you cannot change the currency.

Error message extract from SAP system. Copyright SAP SE.