Don't want to write this by hand? GodCraft sets minecraft:player.level for you with a switch.Get GodCraftOverview
minecraft:player.level is an entity component: it goes in the components object of a custom mob, or in one of its component groups. Stores the player's experience level. Used by the player entity.
Example
Inside the components object of BP/entities/<name>.json (values are the defaults or placeholders):
BP/entities/<name>.json
{
"minecraft:player.level": {}
}