Hello!
Just made a account here because this thing is driving me nuts! 
So, i’m making an accapela and in order for it to work you have to perfectly line up instrumental and original - the problem is, the instrumental is snapping to something and because of that sample points aren’t lined up.
https://imgur.com/a/ZlZNg0w ← That’s what i’m talking about, you can see sample points are not lined up.
How can i fix this?
Thanks! 
So, i’m making an accapela and in order for it to work you have to perfectly line up instrumental and original - the problem is, the instrumental is snapping to something and because of that sample points aren’t lined up.
I don’t know what’s going-on there. If the sample rates match, the samples will align because that’s how they are defined. i.e. At 44.1kHz, the first sample occurs at time-zero and the next sample occurs 1/44,100th of a second later, etc.
Your bigger problem is… It’s highly-unlikely that you have an instrumental copy that’s digitally-identical to the full-production with the exception of missing vocals. In the real world this “never works” and you end-up with a messy-mix of both recordings. When you have two uncorrelated recordings, subtraction usually sounds exactly like addition… Like a normal mix… Mixing is done by summation.
It’s possible to do this if you made the recordings & mixes yourself and you didn’t adjust the levels or otherwise “touch” anything during the mixing. But the odds are, if you have the original instrument track(s) alone you probably already have the vocal track(s) alone.
Dealing only with the sample alignment part of the issue:
If you’re using Audacity 2.3.2:
Now align the tracks using the Time Shift tool.
As DVDdoug wrote, to achieve “cancellation”, the instrument tracks must be identical, which only happens if they derive from the same original source.