The mainframe file assign clause doesn't really assign an actual data file yet - you need JCL to match the file to the program.
Mainframes also have operating systems.
The most common on large IBM mainframes is something called MVS - Multiprogramming Virtual Storage. To run a job on a mainframe system, you need to request and use system resources - you need access to the CPU, data storage, programs, etc. JOB statement introduces the job to the system. The user is 'Bruce White'. Give us all JCL statements and all message.