go to the alignment tab. Then hover with your mouse over area next to FOV. Now a message with the calculated FOV will pop up. Then type the displayed FOV manually in. Now it should work.
Or take a picture and let the plate solver do it’s normal work. It will fill out the FOV for you. Ekos will remember it and next time Polar Alignment can start straight away.
The mouse-over the FOV section for the calculated FOV tooltip only works properly with a saved telescope configuration in the Mount Tab. If you have to change the focal length, you do it in the Mount Tab scope/guide scope configuration and you must save it for it to be applied.
Filling it in manually will greatly speed-up a plate solved FOV value.