You still have not told us your version of Audacity. It’s important that you do that when you need help, or your reply may just be a load of “if” statements that take us a lot longer to write and you longer to read.
If you have 2.1.1 or later, then you can use Vocal Reduction and Isolation to isolate centre-panned vocals. If the vocals are panned otherwise, you should continue to try VoiceTrap or other tools that use other methods.
I am trying to remove the backing but keep the singing part
Note that these kind of effects are rarely “perfect”. They tend to be novelty effects that can be fun to play with but almost never useful in audio production or editing.
“You can’t u-bake a cake or un-fry an egg, and you can’t un-mix a recording”
If the vocals are perfectly centered (identical and in-phase in both channels) you can subtract left from right, and in that case the vocals can be completely removed (along with anything else in the center). And of course, you can completely eliminate the left or right channel (and optionally make a mono track that plays from both speakers).