Depends on what stuff you want to buy. There are level locks on a bunch of things, so you might not have the right level for it. Also eventually happens when you dont have enough RP to buy the stuff you want (if all of that is not part of a bug).
If you wanted to buy an upgrade, then you also have to buy the earlier stages before you can upgrade to an higher one.
In your situation, there is only stage 1 parts to buy anyway, given the fact that you are level 4.
If you are on the "System" channel with your chatbox, then this shows up informing you what things you need to buy, before you can buy the wished ones.