First up… check that there really is something listening on 127.0.0.1:8081 - remember Nexus takes a while to start up and open that port. Something like curl http://localhost:8081/ should return something that looks like HTML. If it doesn’t then check your Nexus log, as something’s not right there (is your VM too small, perhaps?)
FWIW, in my (AWS) setup, I didn’t uncomment the nexus.properties line - there’s no need unless you’re changing it.
The only other thing to try is perhaps to remove the nocanon from the ProxyPass directive. It’s been a while since I used Apache, but I don’t ever remember using that before. I also don’t recognise AllowEncodedSlashes - maybe try without that too, just in case. Either way, if the port’s open and returning HTML, then my money’s on your Apache config.