Getting decode error.

[snippsat]

Like this and you most also activate stdout,stderr pipes.
Now you decode out.decode() is the same as out.decode('utf-8')

def run_process(cmd_args):
    with subprocess.Popen(cmd_args, stdout=subprocess.PIPE, stderr=subprocess.PIPE) as p:
        out, err = p.communicate()
    return out

If also want to return err return out,err,then it become a Tuple and you use res[0].decode().

This is a very old way of doing string formatting.
From 3.6 use f-string.

f.write('Process Output : %s\n' % (res.decode("utf-8"))).
# To
f.write(f'Process Output: {res.decode()}\n')