Yard machine 21'' push mower runs 2 to 3 seconds cuts off.

The dead-man switch on the lever (that you have to hold in to keep it running) has a loose connection or you have a loose spark plug wire.

Bad gas. Drain gas and refill with fresh gas with Stabil. Old oil. Drain oil fill with fresh 30w oil, 16-18oz. Clean air filter and spark plug.. Relpace air filter if dirty. Saving $$$ on this simple tune up is not intelligent.

have 15.5 yardmachine will turn over but not start check spark plug oil filter changed gas runs fine and blade handle up

Drain plug for the oil should be under the deck next to the blade shaft. Usually a plug with a square head on the bottom of the motor. Loosen the fill plug and then remove the drain plug. Any time you go under there make sure you disconnect the spark plug wire so no spark can jump.
Gas is another thing. No fuel line as carb is part of the tank, so only way to empty is to .....
1) tilt the mower over and pour out the gas. Good time to change the oil and do it before you add the new oil.
2) unbolt the tank from the motor and remove tank carb and all a single unit. Then turn it over and dump the fuel. there are only 2 bolts holding it on so after you remover the air filter you can see them easily. Just remember how the throttle linkage hooks on.

Remove the bowl from the carb. and blow it out with air.

Most of these machine will have an oil drain plug under the mower deck near the center of the blade. 80 to 90 % of them are a 3/8" drive female plug that can be removed with a ratchet and a short extention.
When you put it back in it is a good idea to clean it well and wrap the threads with 2 or 3 layers of teflon tape

Some mowers are designed to drain oil out of the dipstick tube. remove dipstick and tilt the mower to allow oil to run out into a drain pan. 2/3 rds of a quart or 20 oz is what to expect.

I’m happy to assist further over the phone at https://www.6ya.com/expert/rolandw_c013176d62ee17ae

Sounds like it time for a tuneup. Air filter changed, Spark plugs replace and oil changed, plus sharpen the blades while you are at it.

Most common is a dirty air filter, followed by a plugged vent hole in the filler cap on the gas tank. Next would be fuel issues either in the supply line or in the carburetor.
Hope this helps, --W/D--

Sounds like you need a complete tuneup. Start with the carb. then clean all coil contacts, air filter and this will start you in the right direction. Good luck.