Question

Please, could someone explain me why "make-array" has no effect on plant1?

(LET (plant1) ((setq plant1 (make-array ‘(4 4))) (print plant1) (setf (AREF PLANT1 0 0) 1)))

NIL
Error: Attempt to do an array operation on NIL which is not an array.
[condition type: TYPE-ERROR]

Answer 1

(LET (plant1) ((setq plant1 (make-array '(4 4))) (print plant1) (setf (AREF PLANT1 0 0) 1)))

First rule: format your code.

(LET (plant1)
  ((setq plant1 (make-array '(4 4)))
   (print plant1)
   (setf (AREF PLANT1 0 0) 1)))

There are too many parentheses. The syntax of let has a body of code, which is a sequence of forms. You have parentheses around the sequence, which is wrong. Read the Common Lisp HyperSpec entry for LET. It mentions the syntax.

Use:

(LET (plant1)
   (setq plant1 (make-array '(4 4)))
   (print plant1)
   (setf (AREF PLANT1 0 0) 1))

Which is the same as:

(LET ((plant1 (make-array '(4 4))))
   (print plant1)
   (setf (AREF PLANT1 0 0) 1))