I want to give an item on join

Well well, there are different types of methods in this case, let's explain both:

Give an item on first join (so the item is only given ONE time)

Edit the giveFirstJoinFeatures more information here https://docs.ssomar.com/executableitems/configurations/item-configuration/item-features#custom-give-first-join-features

Give an item on each join

That is not correctly handled by EI, you can do it only if you have an item that is ALWAYS in the player inventories, something that most servers don't have, the best way to achieve this is using ExecutableEvents.