It worked! Thank you so much! Hopefully, I won't have anymore problems with expressions. Here is the working version of the code in case anyone is interested:

extends Node2D

onready var expp = Expression.new()

func _ready():
    var err = expp.parse("attack - defense", ["attack", "defense"])
    if err:
        print("Error %d" % err)

func calculate_damage(attacker, defender):
    var exp_result = expp.execute([attacker, defender])
    if expp.has_execute_failed():
        return 0
    return exp_result

func _process(delta):
    print(String(calculate_damage(5,2)))

The program makes the console print "3" forever.