With Krastorio 2 mod:

Failed to load mods: Error in assignID: item with name 'fuel' does not exist.

Source: drone-fuel-from-fuel (recipe).

Fixed. The problem was Krastorio 2 changed the internal names of their fuels (likely during the overhaul to make it work in factorio 2.0) and I was using the old names because I released 0.5.0 before Krastorio 2 was released for factorio 2.0. It should be fixed now.

Thanks! It works! Some minor FRs:

1. It required to change in transport drone options drone fuel to 'drone-fuel', that wasn't done automagically, and there is no any message about possible error. Maybe add message to chat, if this mod installed, but using 'drone-fuel' not configured in Transport Drone?
2. complex fuels not produced in drone fuel refinery, and not in a Krastorio fuel refinery, it produced in chemical plants, maybe in Krastorio:
   • enable produce drone fuel (including from complex fuels) in Krastorio fuel refinery
   • unlock krastorio fuel refinery early in "Transport system" technology
3. some ideas for more streamline Krastorio integration
   • add drone fuel from biomethanol recipe
   • add drone fuel from coke recipe

added the error message. I will look in to the rest when I get around to revisiting more complex integrations with Krastorio.