Hex editing problem, please help

[toadboy96]

Well, recently i started playing a 1.12.1 server and i heard cache editing works on these old servers, ive never hex edited before but i recently tried it, ive been going off this tutorial Hexworkshop wow item mod by ~kronixusa on deviantART and i tryed it ingame, i bought the item from the vendor (it was origionally a skinning knife) and it was now a minor healing potion with no use on it. i changed the spell on it to dmg11 which is supposed to do about 3,000,000 damage and when i right click the new skinning knife it says invalid target, i go outside to a bunch of mobs, click one, and right click the new knife and it says the item was not found, what did i do wrong? any help please