What computer operating system is in use?
How are you trying to open the 12 project file in 13?
Are both program currently on the same computer?
Have you moved, deleted, or renamed files and folders that went into the 12 project after you saved closed the 12 project?
Do you have any features of the 12 project that do not exist in 13....one example, if you are using a certain disc menu in the 12 project file, is that menu available in 13 or activated for use in 13?
Let us start here and then decide what next. Any questions or need clarification on the above, please do not hesitate to ask.
Is it possible you're trying to open the .prel file using Add Media rather than File/Open?
Version 12 files usually open pretty easily in version 13 (but then you can't later re-open them in version 12). Assuming of course that you're the same platform. Windows project files don't open on a Mac and vice versa.
The questions are already asked about opening of the project and more. Your answers will help sort out the situation that you are facing.
It is very likely that your error message is not just codec missing but rather non support or codec missing which is a generic type message in this case.
There is no guarantee that project file from an earlier version (no matter how close the versions are) will open in the later version.
One thing that I did not mention is please work with a copy of the 12 project as a safety net due to issues with backward compatibility of project files.