How to fix this

[karmic frost]

If you remove the section responsible for removing flags, there is no error?

and what version are you using

you can either use my solution or the one from skbee wiki
https://github.com/ShaneBeee/SkBee/wiki/Tricks-Hide-Attribute-Modifiers

import:
    org.bukkit.inventory.ItemFlag
    org.bukkit.inventory.meta.ItemMeta
function hideAll(item: item) :: object:
    set {_meta} to {_item}.getItemMeta() 
    {_meta}.setAttributeModifiers({_item}.getType().getDefaultAttributeModifiers())
    {_meta}.addItemFlags([ItemFlag.HIDE_ATTRIBUTES])
    {_item}.setItemMeta({_meta})
    return {_item}

https://jd.papermc.io/paper/1.21.8/org/bukkit/inventory/ItemFlag.html
here you have all possible hiding options if you don't want to hide everything at once