forge-1.18.2-40.2.4
[minecraft/LootTables]: Couldn't parse loot table guardvillagers:entities/guard_helmet
com.google.gson.JsonSyntaxException: Expected name to be an item, was unknown string 'minecraft:gold_helmet'

thanks，so,what should I do?I'm a Novice，and my en is so bad

this problem is caused by the fact that you miswrote minecraft:golden_helmet as gold_helmet.

i recommend you use this site for generating the guard loot tables, though it may require some tinkering

https://misode.github.io/loot-table/

thanks,I will be trying