Squeeze the throttle trigger, move the master control to the choke position.
Slide the lock to the rear.
Remove the shroud up and to the rear.
My guess is you are not depressing the choke lever first?
If you did not buy new, see if you can find online or download a copy of the 171, 181, 211 owners manual.
If you did buy new- read the owners manual.
Should be on about page 45 of the English version.