RC12 occurs when System come across some problem which it was unable to understand or because of other reason which system cannot interpret.
Common issue: • Import is canceled due to object missing.
• Objects are not active.
• Program terminated due to job "RDDEXECL" is not working.
• Connection problem between system.
These issues may be generally solved by clearing the import queue and again transporting the Transport request.
But in case if this error comes again and again during transporting transformations this may be due to some orphaned transformation rule IDs.
But sometime
even after clearing the queue TR give you the same error RC=12.
I have face this issue with transport of transformations from one system to another.
This is because when we create some transformations, with or without routines or even in one to one mapping SAP system creates rule per mapping.
You can check this mapping in table RSTRANSTEPROUT.
Against every rule ID there is an ABAP code (even that rule is constant/Routine/one to
one mapping/etc.)
Now in some cases because of frequent correction in transformation (that we generally do
because of change in logic or bugs), these rules get erased from this table.
But this doesn’t give any error in TRFN while checking or activating them.
Check the link: http://scn.sap.com/thread/3406781
RC12 error in transport looks like:
We can go to SM37 and check the cause of error
Output of SM37:
This doesn't give any insight of what is happening because system itself don't know 
For checking that you are facing some ABAP rule deletion issue you can go to table RSTRANSTEPROUT
Enter your Transformation ID and execute, you will get something like shown below
Check if ABAP rules are present for all the transformation ID & rule ID.
IF some rules are not present like shown above (Rule ID 30 &40), we need to delete this transformation ID (row) from this table
Now there are two methods of doing this:
- Go to SE80: Class: - CL_RSTRAN_STAT
You will get several programs there, select METHOD 'Delete_rule_from_db' and execute.
Fill the required Transformation’s technical name and execute to delete orphaned transformations
from system.
2. Program: RSTRAN_ROUT_RSFO_CHECK
Go to SE38 and execute this program
Give transformation ID
Version can be A if your transformation is active or M if it is modified version.
First simulate this program, and check that if there are any issue in your transformation.
Repair if errors are found , this will delete all the orphaned rules and your transformation will be fit to go.