How to make this unbreakable - Minecraft 1.14.2 replaceitem Command [duplicate]

It doesn't actually matter where you put it, as long as you don't put it into a different tag.

These should all work:

/replaceitem entity @a container.0 crossbow{Unbreakable:1,display:{Name:"\"Paintball Cannon\"",Lore:["\"Put the fireworks in your offhand, and watch them go BOOM!\""]},Enchantments:[{id:quick_charge,lvl:3}]}
/replaceitem entity @a container.0 crossbow{display:{Name:"\"Paintball Cannon\"",Lore:["\"Put the fireworks in your offhand, and watch them go BOOM!\""]},Unbreakable:1,Enchantments:[{id:quick_charge,lvl:3}]}
/replaceitem entity @a container.0 crossbow{display:{Name:"\"Paintball Cannon\"",Lore:["\"Put the fireworks in your offhand, and watch them go BOOM!\""]},Enchantments:[{id:quick_charge,lvl:3}],Unbreakable:1}

It is important that the U in Unbreakable is capitalized.