There are different categories for levels, you have your player levels which you increase by gaining XP, each weapons has a level and the Battle Pass rewards you with unlocks of things like skins, operators and Blueprints.
The 20 levels you have unlocked are for the Battle Pass, I suspect that you are trying to add an attachment to a weapon that will unlock when the weapon reaches level 11, play the game and shoot people with that weapon.
If you check the camo section in your loadout editor you will see that there are different camo types, there are 10 different patterns in each.
You can unlock new camos for achieving targets and unlocking all the camo patterns will give you a gold camo.
Use your weapon with no attachments until you complete all 10 designs from that camo type, it will help you master the weapon faster before adding attachments.
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Question is where is the stuff?
I just noticed that I didn't answer your main question, and it's a really good question, I think given enough time you would have figured this out like I did, but to shortcut that learning curve for you, the game makes it easy for you to see when you have been awarded something new, it does this by highlight the menu in which you will find that new item with a small green square to the right of the menu.
To find everything just follow the green squares, it tells you when you have invites or message, new attachments on weapons, new camos, emblems, charms and weapon perks etc...
Take a little time exploring every menu, it's like peeling an onion, I had the game for a good few weeks before I started to understand just how many layers there are and learning to follow the green icons was one of the key ways that I came to discover this. So go get your greens, they're good for you.