there is a piece next to the flywheel tha has a magnet and grabs resistence that may need replacing. I had this happen on mine. call your local repair shop and ask. I found the answer to mine on youtube and replaced it in about an hour,
Usually a solenoid or battery issue. may be the flywheel too. It does turn and the flywheel on top spins horizontally, yet it will not catch? if so then it may need a solenoid next to the flywheel which grabs resistence as the flywheel spins. cheap fix and on youtube.
Question edited for a lot more clarity, add maker, fix typos, change into what I think you were saying. Manuel is the name of a Spanish gentleman, 'manual' is the word you need.
Manuals in the link, plus the maker. This link show 'M Series' belt.
https://www.manualslib.com/manual/1865101/Lawn-Boy-M-Series.html?page=153
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