There is no unique transfer for the IDoc

What causes this issue?

There are already several transfers to which the IDoc could be assigned.
Only a maximum of one transfer should exist that is suitable for
including the IDoc.
Suitable transfers are transfers that are not closed, that is, for which
the processing step has not yet started or for which the target number
has not yet been achieved.
The following internal transfers would come into question for the IDoc:
&V1&
&V2&
&V3&
&V4&

System Response

The verification step does not assign the IDoc to a transfer and
interrupts processing. The IDoc ends with status 51.

How to fix this error?

Close the transfers, for example by starting the processing step so that
only one transfer remains that can include the IDoc. You can process the
IDoc again afterwards.

Procedure for System Administrators

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.