How I add a new slot to player inventory?

custom code or not doable

As @KantroActual has said, you have to use your own code because adding new slots can cause incompatibilities between mods, so Pylo doesn’t want to add this feature.

If you really want to add a slot you could maybe make a custom GUI that works the same as the player inventory and override the keybind that usually opens the inventory. That would probably be more work than it's worth, so a better solution would probably be to have a separate inventory that is opened by a different key where you can insert your custom jetpack/backpack.

@willk55 i tried this one after i put the item in the gui slot when i exit the gui the item is droped back

this is good, but how would one actually put the armor onto the player if you've already created the extra slot?

Can this GUI be bound to the player? Can its slots count as armour slots? If I added an armour item to the slot would the player count as (LivingEntity wearer)?

you could have a global variable for every slot and same for armor and have a procedure which will save and load slots and you even add the crafting area by using slots and you can have the main hotbar, armor and off hand be the same as a said custom one