How do I troubleshoot a newly installed non-working outlet?
Some breaker panels have blank slots at the end, which could mechanically accept a breaker, but their tabs are not live, if they have tabs at all.  This is done to lower manufacturing costs and reuse one chassis with different bus bar assemblies.

It could also be a piece of tape, clear insulation, or severe corrosion over that spot on the bus bar.  Depending on the resistance, it may let a charge large enough to measure get through, but when an real load is placed on it, the resistance is too high for it to run.