So we're back to the issue of whether or not you can talk to the 8903B. We're never verified that.
First step, open the "Keysight Connection Expert" and find your instrument. Mine is GPIB0::8::INSTR (Yours should be GPIB0::28::INSTR) and then right click on it and select "Check Connection Status":
Once it has checked, the box around the icon should go green if it can communicate. If it is green then right click again and select "Send commands to this instrument". That will open the Keysight Interactive IO utility for your instruments address:
In the Interactive IO utility, click the "Device Clear" button - You should hear the unit click and the front panel should look like this (with nothing connected) - Note whether the Remote and Addressed annunciators are lit:
The type M2 into the command text box and send it - You should hear a soft click and the unit should look like this - Note the Remote and Addressed annunciators should be lit and the SINAD measurement selected:
If the above doesn't happen then there is a fault in either the cable, the 82357B or the GPIB port on the 8903B. I would start by swapping out the cable if you're using one. If the 82357B is directly connected to the 8903B then you should ensure that it is properly seated and the contacts are clean. If that is the case and it still doesn’t work then I would get another GPIB adapter and try that - Anything will work as you're just testing whether it is the 82357B - You could use an NI card, a Prologix, An Arduino adapter, etc.
If you have swapped all of this then the issue is in the GPIB part of the 8903B - The service manuals can be downloaded from the Keysight website.
TonyG