It is not possible to fully, cleanly remove the vocals on Mixxx. In fact it's pretty impossible to do it at all - it's a bit like trying to extract the sugar from a cup of coffee once you've stirred it in. You have two options if you're happy with an okay-but-not-amazing result:
1. Use a program like Audacity, which has a fairly primitive built-in vocal remover. It's not clean or perfect but it can be reasonably effective.
2. Fiddle with the equaliser settings until you hit a sweet spot where reducing the MID band effectively removes the vocals but not the track.
The better option is to trawl through dedicated DJ sites, shops, etc. and see if you can find an a cappella or instrumental version of the track you are looking for.