Document class &1 cannot be assigned to Document type &2.

How to fix this error?

,,Document class provided is assigned to a VAT register different from
the VAT register assigned to current Document Type. Ensure the Document
Class is assigned to only one VAT register. Document class can be
assigned to multiple Document types in the configuration Assign Document
Classes to Document Type but those document types must be assigned to
the same VAT register in the configuration Assign VAT Register to
Accounting Document Types

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

Error message extract from SAP system. Copyright SAP SE.