Object type & is not assigned to application &

What causes this issue?

You have entered a business object type (&V1&) which is not assigned to
application &V2&.

System Response

In system table T681Z a business object type is assigned to both the
header and the item of the application document of application &V2&.
You can enter any of the object types from the business object
repository, which have a key structure consistent with that of one of
the object types.

How to fix this error?

You can see all the permitted object types by pressing the possible
entries pushbutton.

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

System Response

How to fix this error?

Error message extract from SAP system. Copyright SAP SE.