I just reached lvl 7, and received a notification saying that i have received my first arsenal crate, and it has disappeared . I have seen threads on reddit of people saying you get one at lvl 8 not 7, but the notification clearly said I have received one, I even had the little tag on my profile that said i could quick open it, but when i went to the barracks to open it…GONE.
Arsenal crate dissapeared
Kirays
#2
Hello. 
You receive your first arsenal crate at level 5 which remains unlocked until you reach level 7.
After that you always receive an arsenal crate with every level you achieve.
If you do not have it please contact support via the following link.
Make sure to check your filters first as it might just be hiding.