I remember having this problem with the Uzi when Resistance came out. I'm no expert, but I think it happens when you're missing the appropriate addon declaration in the mission.sqm file. For example, the top of one of my mission.sqm files looks like this:
version=11;
class Mission
{
addOns[]=
{
"bis_resistance"
};
If the bis_resistance part is removed, I get the error you mentioned if I pick up an Uzi.
I believe that the official addons will be automatically added to your mission.sqm file if you preview your mission and grab the weapon during your preview. (Don't forget to save after you preview.)