Your post doesn't say, but I assume you have a Briggs & Stratton engine.

Briggs uses a starter clutch screwed on to the end of the crankshaft. Clutch also serves as a nut to hold the flywheel on. Clutch has a series of steel balls and a pawl which allows it to function as a one-way clutch. It's torqued to 65 ft/lbs and requires even more to breakaway.

Overtime, rust, dirt, gum and other crud prevent the clutch pawl from rotating freely on the crankshaft stub.

You need to remove the clutch, and polish the crank stub so that it is smooth and bright. Best fix is to replace the starter clutch 399671 or 495970 with a new one (about $15). Not as good, but cheaper is to disassemble the clutch and LIGHTLY oil (one or two drops) the pawl and reassemble. Some people say not to lubricate the clutch shaft; personally I have had good success with a small drop of oil over the past 30 years of small engine repair.

To do the job properly, you really need a clutch wrench and a a way of keeping the flywheel from turning. If you have an impact wrench, Briggs 19244 Clutch Tool (about $12) is sufficient. If you are turning by hand, you need to restrain the flywheel, and best tool is Briggs 750075 Flywheel Holder (about $30).