What Craigav suggest could be the issue, i.e., low water flow (gallons per minute (gpm)) will not turn on the tankless water heater.
However, there's an easy test to see if that's the issue. Turn on the bathroom skin's hot water faucet... once it's hot, keep it on while also turning on the shower to full hot setting.
If no hot water at the shower and you have hot water at the sink, then try this.... leave the sink hot on, but the shower faucet set to the coldest setting.
If you still don't have hot water at the shower head after you try the above, you just confirmed it's not your hot water tank.
If you do have hot water at the shower, then keep it running and turn off the sink hot water... wait a minute or so and see if the shower remains Hot or turns to cold..... if it goes cold, then most likely its low gpm water flow. Another test you could try is remove shower head only and try again with the higher gpm water flow. If the Hot returns, change your shower head
Lastly, if you got hot water at the shower head when the faucet was set to cold, then shower faucet is plumbed wrong, e.g., cold to hot and hot to cold.
If none of the above yields no hot water at the shower head, try what I posted first, i.e, the scald valve setting.
2021 Precept 34G, Brevard, NC