My Hayward H400FDN heater will not switch out of "STANDBY" mode when I press the mode button. Any Ideas on how to fix?

Possibly bad control board or a bad connection, need a little more info and I only have my phone to answer questions so I don't see the type of heater your posted under if you did.

make sure wiring is correct especially if wired 120 volt read instructions inside panel

This is a normal display when heater is being controlled by a remote thermostat. No service is required. If heater is not being controlled by remote thermostat, change setting by using the MODE key to put the heater into STANDBY. Press and hold the DOWN key and then press and hold the MODE key. HOLD down both keys for 3 seconds until the indication "bo" is removed from the display.
The key to this is DOWN first and then together with MODE, both for 3 seconds.

Make sure the display is not showing "BD".

I had the same issue...you don't have enough gas pressure.