Probably contact Influence is too low. You can see what levels of weapons are available if you click/tap on the weapons service before clicking/tapping the buy/sell options.
"Probably contact influence" My point exactly... As a new player i have no idea what she is saying... Arg... tell me what you need... do you want roses??
If the Contact doesn't have enough Influence, it means they don't have the connections and wealth needed to get those weapons for you. Sell them Intel, run missions for them, or look for their friendly connections and run missions for their friends.
The reason they won't sell you weapons is shown right on the same screen, unless this is another screen where mobile has to hide some information. It will tell you what contact influence level is needed. Basically, they trust you a lot, but their contacts don't trust them, so they can't get any deliveries of weapons to sell you.