You need a flight plan in the CDU.
After selecting the IRS switches to NAV, in the CDU, you must start the IRS alignment by clicking the message displayed at LSK 4R on the INIT page.
Don’t move the aircraft until the alignment is complete.
You must ensure the VOR/NAV/ILS switch above the PFD is in NAV
Press Nav button on AP.
Ensure AP is on.
Here is a link to the real world A310 manual.
Lots of good info there.
https://www.smartcockpit.com/plane/AIRBUS/A300-A310.html