Master appl.tech offers these tips:

Ensure your filament leads on the magnatron aren't open (open = bad)

need to verify that your getting 120V to the inverter (if not = bad board)

the 2 signal wires to the inverter should be showing 3VAC from the board (if not = bad board)

If the above all check good then you have a bad inverter.

Source: Panasonic Microwave NN-SD277W