thats right because ur rank is not high enough to buy a capsule you need to have a legit rank to buy it if its faked it just shows an other rank but it isnt working to buy capsules or weapons for higher ranks
The actual reason is because you do not have enough Euro to buy the capsules. If your end Euro amount after you buy the capsule is less than a certain amount (i believe it's 20k) then it will not allow you to buy the capsule.
For example: You have 25k, and you buy 1 capsule for 1k. It will allow it because your end Euro amount will be 24k. Now let's say you only have 20k, and you try to buy a capsule for 1k. It will not allow it because your end Euro amount will be 19k. (NOTE: THIS WAS JUST AN EXAMPLE).