I disabled all but the base mod. I enabled the mod and only it's requirements.

126.527 Checksum for script Hiroshima-Scenario/control.lua: 3795083680
126.558 Error MainLoop.cpp:1284: Exception at tick 0: The mod Hiroshima (Scenario) (0.1.0) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event Hiroshima-Scenario::on_player_created (ID 26)
Hiroshima-Scenario/freeplay.lua:68: attempt to index a nil value
stack traceback:
Hiroshima-Scenario/freeplay.lua:68: in function 'handler'
core/lualib/event_handler.lua:47: in function <core/lualib/event_handler.lua:45>

Did you get this error while playing the scenario or were you doing some other action? (edited for clarity)

On load. It crashed and went back to menu

Upon closer investigation, I can only replicate that crash when creating a new freeplay game with the Hiroshima Scenario mod/modpack enabled. Did you ensure that the unzipped Hiroshima-(Scenario)_0.1.0 was in the .../scenarios folder and the contents were outside of another folder?

Ex. .../scenarios/Hiroshima-(Scenario)_0.1.0 instead of .../scenarios/Hiroshima-(Scenario)_0.1.0/Hiroshima-(Scenario)_0.1.0

The mod extracts to the mod directory, not the scenario directory. Does this require manual installation?

To a degree, yes. A current known issue is when this mod/scenario is set as a scenario, it will error upon trying to load. You need to copy the downloaded zip from .../mods to .../scenarios and unzip it. When running the scenario, please ensure that you have the mod enabled.

If you need more steps, please look at the updated description. It has step by step installation instructions.