Before turning on the water heater, be sure the propane lines are purged of air. Operate a couple of the stove burners for a few seconds to ensure that propane is completely filling the propane lines and manifold.
Also, did you open the door on the water heater and turn the switch ON. That's on page 11 and 12 of the manual at the link below.
Girard Tankless Water Heater Operator's manual:
https://www.manualslib.com/manual/63...-1.html#manual
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