OK this isn't my answer I got it from another site.
However the suggestion is that your selector lever has probably rounded off, which is why it won't select.
And that maybe the transmission is done. Apparently all parts are available. I'll post what was written so you can check it yourself.
I don't know of any detailed instructions. Take the tranny out, take the pulley off the top. If the tranny is caked up with debris, clean it off so you don't get the mess inside it. Take the case bolts out all around the tranny. With the tranny front facing you on the workbench, gently lift and pry the top of the case upwards, being careful to make sure all the parts other than the shifter connection and input shaft stay in the bottom half. Once the top is off, the shaft closest to you with several gears that increase in size... that is the shaft you'll need to pull out. Take it out and slide each piece off and lay it on the bench in the order it was removed, with each piece face up. There will be 2 or 3 grooves in the shaft, and that is where the shift keys are. Slide the shift collar off and the keys will come with it. Install the new keys and reverse procedure to reassemble. The old keys are probably rounded off at the tabs on the ends. Check the gears as well, where the keys latch and make sure they aren't worn too far, which will allow the new keys to slip.
Once you get the shaft back together, fit it back in place in the lower half of the tranny. Add some "00" grease if needed, and make sure the bushings on the shaft are lined up with the flats on the bushings matching with the flats in the case, or you may break the case when you tighten the bolts. Sit the top of the case on and line the shifter up with the shift ring. Some models require this is done prior to installing the top if the shifter is a fork rather than a single button type. If the top case half fits all the way down tight and flush with the bottom half, bolt it back together. If it doesn't. open it back up and figure out why. Usually it's one of the bronze bushings on one of the shafts, or a thrust washer over too far and in the way. Get the make and model of the unit and look up a diagram if needed, in case you get confused.
[https://www.doityourself.com/forum/outdoor-gasoline-electric-powered-equipment-small-engines/472723-kgro-power-pro-model-lt4218-gear-problems.html|original post]