OCaml interpreter functions

I am looking for assistance with creating specific interpreter functions for my OCaml project. Specifically, I need help with the 'bind' and 'let...end' functions.

Requirements for the 'bind' function:
-The bind command binds a name(where name consists of a sequence of letters, digits, and underscores, starting with a letter or underscore.) to a value.
-It is evaluated by popping two values from the stack.
-The second value popped must be a name
-The name is bound to the value (the first thing popped off the stack). The value can be any of the
following:
• An integer
• A string
• A boolean
• :unit:
• The value of a name that has been previously bound

Requirements for the 'let...end' function:
-It should create a separate stack after 'let' is read from a txt file
-It should push the top of the stack to the original stack after the keyword "end"
-Trying to access an element that is not in scope of the let...end block would push :error: on
the stack.
-let...end blocks can also be nested.

Below I've attached the ml code I have so far. Everything is correct except for possibly the let function which I have as a comment(line 316)
I've also attached two test files that have to have the output that is shown in the. other two files.
Message me if you have any questions!