Plastic PVC collar won't catch threads...

This is probably very simple for most of you but I'm a super-amateur. I had to replace the pipes under my kitchen sink due to leakage. I've gotten all the proper pieces (I took it apart and took it with me to the hardware store). Now, I'm trying to screw a collar onto an existing piece of PVC. However, the collar seems to be just slightly too large to catch and hold onto the threads. Everything else fits perfectly and is ready to go. Any advice? (the u-shaped piece of PVC came with the collar already on it - I didn't pick it out separately. Looks like it's perfect, just doesn't quite fit).
THANKS!!

I think you are calling the compression nut a collar. Underneath that is a tapered plastic or rubber tapered ring. The nut tightens down and pushes that tapered ring into the small gap around the pipe, creating the actual seal. Sometimes, you have to wiggle it a little to get that ring lined up and into that small gap. That should allow you to engage the threads. If it is not that, I don't know...

Some of the really cheap versions aren't made really well, and when the threads are cut or molded, they aren't the right size, and nothing will fit properly.

nut

If the plastic washer is upside down it can interfere with installing the nut, but sometimes the nut is too large, the thread is too small, or the nut is defective. Try a different nut and see how it fits.

Sounds to me like someone bought 1.5" when they needed 1.25" or v/v.