Document type not changed (choose another document type)

What causes this issue?

The function for changing a document type was selected. However, a new
document type was not specified but the old one entered again.

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?

Choose a new document type or leave the transaction.

Error message extract from SAP system. Copyright SAP SE.