External number assignment only possible with print transaction

What causes this issue?

You attempted to issue an output for which an external document number
assignment should be carried out, according to the print control
settings. Outputs with external number assignments can only be issued
by using the print transaction in foreign trade outputs, not by using
other methods, as was the case in this situation. These other methods
could include:

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

issuing an output as you post billing documents via send time '4'
using other print transactions
issuing an output via RSNAST00

System Response

The system cancels this attempt to issue an output.

How to fix this error?

Use the print transaction in foreign trade outputs to display or print
this output.

Error message extract from SAP system. Copyright SAP SE.