"You must turn left here" was not accepted. Reported. Also Duolingo is doing its usual thing with contractions and saying "You've to turn left here", which is wrong
It's now accepted. Thanks for the report.
The contraction problem is not a contributor problem, but a Duolingo problem. They do that thing with all their courses involving English.
Why not "you should" ?
why not you must
It is accepted if the rest of your sentence is correct, e.g., "you must turn left here". You may also say "you have to", "you have got to", "you need to".