How to fix this error Idk

Yes, if you want to prevent a level 1 from being created without a corresponding level 0 being available, then you need to do a crosscheck in the script so RMAN knows that the level 0 is not available.

Prior to 10g (or with compatibility < 10.0) Oracle would do a level 0 when a level 1 was done without a level 0. Since you are on 11g, I would expect your compatibility to be >=10.0, so RMAN "copies all blocks that have been changed since the file was created". This behavior seems to have changed between versions and there is some confusion as to what the current behavior is, but your findings are consistent with the documentation.

Are you sure this is an RMAN backup set? Maybe it is just a dump file made while the database is online. For 9i you only had exp and not the newer expdp available to make dumps so you could try to import it with imp and see what the output is:

imp system/password@abc FULL=Y FILE=file_decrypted_379299.bak