why i cant buy weapon !?
i have money and when i go to buy it it tells me; 'failed to buy this item'

Be patient they're working on a fix.

There is a bug in the latest patch. You'll get a disconnect if you try to modify a weapon. You can also not buy weapons if you sold something before that to get more euro's. Like almar said, the GMs and the developers are aware of this issue and shall be fixed in the next update.

Yep, issue isn't fixed yet but it should be fixed really really soon on the upcoming update.