I use a Nikon D3300 for nightclubs with a 35mm 1.8 lens and a Yongnuo YN-460 speedlight. Most of the shots I take are out of focus or the shutter doesn't work, saying the subject is too dark. I've used the in camera AF assist light but I still don't get good results. Should I buy a new Yongnuo speedlight with an AF assist? or any other suggestions?
Depending on the size of the nightclub and how you're using the speedlight, one of the main issues you've got is that you're using a YN-460, which is a manual-only speedlight that does not automatically adjust the power level to match the scene being metered (i.e., doesn't do iTTL), and that you're probably shooting in an automated mode which is stopping the shutter from releasing when the shot is too dark/out of focus.
I'd suggest learning to shoot in M mode, so that you can tell the camera you're fine with most of the frame being dark. A flash with an AF assist lamp might come in useful, but may also be far more annoying to the folks around you. A good flash, if your subjects or bounce surfaces are nearby could, however, allow you to use a smaller aperture setting, like f/4, that wouldn't be nearly as hard to autofocus accurately as using f/1.8 would, and in a pinch might let you zone focus by using the focus scale and manual focus ring.
Just my opinion, but the YN-460 is the wrong tool for shooting events/social. It's much easier with a TTL-capable flash, and that's only if you're shooting nearby subjects and don't mind the background being mostly dark. If you're trying to light up the entire club with a hotshoe flash, that's not gonna happen. Speedlights are still only powered by 4xAA batteries.
A good (low) budget for a TTL flash is around US$200. Going cheaper than $100 is inevitably going to involve compromises. Yongnuo gear is good for off-camera manual shooting. Not so great for on-camera run'n'gun bouncing. I'd recommend considering a used SB-800 or similar flash if you can find one. Or, if you're really strapped, maybe looking for an old used SB-24 or SB-26, which at least have an autothyristor mode.
I'd suggest using manual focus. If that isn't working well for you, take more shots or use a smaller aperture(I.e. f/4 instead of f/2.8) and a higher ISO.