If the bulbs check good, replace the flasher relay.
Your taillight is burned out.
You have a failed bulb on the right side.
Bad ground.
Normal cause is failed bulb.
Replace the turn signal flasher relay.
If the right one works,it might be a bad bulb or a bad ground.
Sounds like you need a new flasher.
Probably needs a turn signal flasher
check the bulbs one is burned out
Because you need a new bulb.
Sounds like the signal/4 way switch has a short.
Check for; a shorted or bad bulb bad ground in socket