Just remove the parenthesis when the function calls itself:
let my_map ff list =
let rec mapAcc ff_inner list_inner acc =
match list_inner with
| [] -> acc
| front::rest -> mapAcc ff_inner rest (ff_inner(front) :: acc)
mapAcc ff list []
otherwise everything contained there is interpreted as a single parameter and ff_inner
as a function call with the rest as parameters.