Ended with return code: ===> & <===

What causes this issue?

Meaning of the return codes:
0: Transport successful.
4: Warnings occurred.
All the objects were transported successfully. Peculiarities occurred
for certain objects. These might not have been intentional.
<ZH>Example:</>
A warning is given during a transport if the request contains an object
deletion.
6: Post-processing required
The transport request contains documentation objects that were not saved
in an active form, and therefore could not be transported. To transport
these objects, you need to include them in a new request and save them
in active form.
8: Transport carried out with errors.
Individual objects could not be transported successfully.
<ZH>Examples:</>
- The original object was not overwritten
- A repaired object was not overwritten
12 and greater: The transport was terminated
A serious error occurred which normally is not caused by the contents of
the request.
13: The transport tool was terminated by the operating system with a
signal.
14: The transport tool terminated for an unknown reason. See Note 43552.
16: The transport tool terminated due to an internal error.
Other less frequent return codes:
The appropriate transport tool passes the above-specified return codes
to tp. If there is an unexpected reaction by tp when the request is
handled, tp might change this return code by adding a multiple of 100.
The last two digits of the exit code are not changed by this addition.
For this reason, if return codes greater than 100 occur, you should
evaluate the last two digits as described above.
Example:
1204: Warnings occurred.
All the objects were transported successfully. Peculiarities occurred
for some objects. These might not have been intentional. You must
evaluate the first two digits of the return code separately.
Meaning of the additional return codes (multiple of 100):
1000: At the time of the transport, the setting in the transport profile
was such that requests of the corresponding transport type could not be
transported.
1100: Is no longer used. This return code was used when exporting C
sources whose predecessor was not yet exported.
1200: At the time of the transport, the transport profile was set so
that the transport request could not be entered in the buffer of the
target system. The request therefore is not automatically marked for
import.
6000: The target system is not defined. The request could not be marked
for import to the target system.
8000: The request could not be marked for import to the target system.
Usually problems occurred when describing the corresponding file at
operating system level.
9000: The request was imported with unconditional mode 0 (overtaker) and
must be imported again at a later time.

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

&WhAT_TO_DO&
4: Read the warnings.
8: Find out the cause and correct the error.
12 and greater: Inform the system administrator.

Error message extract from SAP system. Copyright SAP SE.