What you are seeing is error code 12 which is a flame failure. A flame failure can be caused by many things. Rannai has a list of trouble shooting tips. This may or may not be something that you can remedy yourself. Here is the list and you can decide which you would like to try on your own but there is a high possibility that you will need a service technician. If your tankless water heater is less than a year old parts and labor repairs will be covered. Here is a list of things that you can try if you feel up to the task.
Check for obstructions in the flue outlet.
Ensure gas type and pressure is correct.
Bleed all air from gas lines.
Ensure proper Rinnai venting material was installed.
Ensure condensation collar was installed properly.
Ensure vent length is within limits.
Verify dip switches are set properly.
Ensure appliance is properly grounded.
Disconnect all MSA controls if installed.
Check power supply for loose connections.
Check power supply for proper voltage and voltage drops.
Ensure flame rod wire is connected.
Check flame rod for carbon build-up.
Disconnect and re-connect all wiring harnesses on unit and PC board.
Check all components for electrical short.
Check gas solenoid valves for open or short circuits.
Remove burner plate and inspect burner surface for condensation or debris.
Let me know if I can be of further assistance.