Why is the /enchant command giving me an error when I go outside the normal level bounds?

If you want to enchant an item with very high enchantments that are not supposed to be allowed, like sharpness 6, you'd need to use /give. For example, for a sword with sharpness 12 and knockback 5, you'd need to use:

/give @p diamond_sword{Enchantments:[{id:sharpness,lvl:12},{id:knockback,lvl:5}]}. 

You can enchant just about everything, so if you want a stick that has a very high knockback, you can simply use:

/give @p stick{Enchantments:[{id:knockback,lvl:30}]}

You can replace these enchantments with any enchantments, for example, you can use:

/give @p diamond_sword{Enchantments:[{id:smite,lvl:65},{id:looting,lvl:10000},{id:fire_aspect,lvl:12}]}

or:

/give @p stick{Enchantments:[{id:sharpness,lvl:130}]}

If you want to get an enchanted pickaxe, you can use:

/give @p diamond_pickaxe{Enchantments:[{id:fortune,lvl:1000},{id:efficiency,lvl:10000}]}

You can do this with any enchantments you want, and almost any object you want.