or "But." Paul is saying that freedom does not include "an opportunity for the flesh."
## an opportunity for the flesh ##
"a chance to do what pleases your(plural) sinful natures," specifically things that harm one's self or neighbors
## the whole law stands fulfilled in one commandment, in this ##
This can mean 1) "you can put the whole law into just one commandment, which is this" or 2) "by obeying one commandment, you obey all the commandments, and that one commandment is this."