What's wrong with my attempt to add 1 to every element in my list in Ocaml?

I don't understand the error message I'm getting or what's wrong with what I'm trying to do

I just want to use List.fold_left to apply my add1 function to this list [1,2,3]

My add1 function should just add 1 to each element, so I would get [2, 3, 4]

My main goal in doing this exercise is just to experiment with List.fold_left. I don't actually care about adding 1, I just choose that function because it seemed easy to write (I'm an ocaml beginner).

My ultimate goal is actually to populate the keys of a empty StringMap using List.fold_left and a function already written elsewhere, so if anyone has insight on that it would also be appreciated

Here's the 1st try (which I tried twice)

let rec add1 = function  | [] -> []  | h::t -> (h+1)::(add1 t) in List.fold_left add1 [1, 2, 3];;

Here's the 2nd try

 let a(b) =let rec add1 = function  | [] -> []  | h::t -> (h+1)::(add1 t)in let c = List.fold_left add1 bin a [1,2,3];;