Can't remove crank arm on older Mercier I'm rebuilding an older Mercier and have decided to tackle the bottom bracket. I've read a few articles and watched a few videos on how to remove the crank but none of them look anything like what I've got on this bike.

Basically, there are bolt threads on one side and a 'nub' on the other side that would indicate that this is what I would remove the crank arm with, but I'm not having any luck removing the bolt.

I've attached a picture as I'm sure I'm probably not using the correct terms for some of these parts.
What you are looking at is called a "Cottered Crank". The shaped cotter pin holds the crank in place and they can be next to impossible to remove without significant effort or a cotter pin press.

Sheldon Brown has an excellent article on removing cottered cranks using a hammer and pipe, which is a method I have used before, albeit multiple decades ago.

When you get it off you may want to consider replacing the bottom bracket and cranks with a more modern design that is easier to maintain, unless this is a restoration job and you want to stay true to the original.