For a look into the debugging process, the important text here is
The method named :* with args [10] doesn't exist on [NilClass, NilClassFalseClass].
(The keywords being the method name, its args, and what it's supposed to exist on. So here, I'm apparently multiplying nil by 10 for some reason.)
And even more important, the "backtrace" pointing at line 201 in the "enemies.rb" file, which is
char.health = 40 + char.hp_level * 10
Which implies that when the game checks "char.hp_level", it gets back "nil", meaning mostly likely this isn't getting set. This is weird because all the characters are supposed to have an HP level, it should be 0, 1, 2, or 3 (meaning they have collected 0, 1, 2, or 3 HP upgrades, which means their max health is 40, 50, 60, or 70)
So the solution is some rigorous debugging to figure out exactly why the hp level seems to be missing