If the FMC asks you t enter the IRS Position again, just click on LSK R6 (Line Select Key    Right side, number 6) to insert it again. It sometimes has a habit of not accepting it the first time.

@Jazz777. You should, but you can do it later, especially if it doesn't accept it the first time, but if the aircraft doesn't know its own location, it can't compute the fuel usage or ETE/ETA, etc., so the PROG page will look incomplete.