How do I execute a PL/SQL package from my database SQL> prompt?
You cannot execute a PL/SQL package, as it is simply a container for one or more routines (stored procedures and functions). Typically you use packages to organize various related routines.

You execute (call) individual routines in a package by referencing them by their names, e.g.

exec PKG_NAME.SPROC_NAME


or in your case

exec C_FIX_STUCK_COS.FIX_STUCK_COS


as both package and SP have very similar names (which is not necessarily a good idea in itself).