Where is the oil filter on yardman 13AC76LF055

Its to stop the dust blocking the filter. The filter should be cleaned with petrol every 4 to 6 months to remove the dust & oil build up, cleaned in/with petrol and re-oiled. Its also good to clean the filter housing each time you clean the filter

I suggest that you should remove deck, turn upside down and place straight edge or tight string line over blades from outer edges of deck frame. Use the guide strips to get even measurement. Rotate blades and measure distance from straight edge to tips at both sides. they should be the same. If there is difference from one side to the other, either the blades are bent, or if each blade is same at each end but not aligned in centre (difference in height) then the spindles need packing to make them even. Check that bearings are not loose. If you do not understand this best to take it to service centre.

You haven't supplied a lot of information but generally when this happens it is due t the mower being tipped the wrong way.
If you must look under the mower then tip it back with the handles to the ground and front wheel in the air.
Now to rectify your problem; The air-filter will probably be saturated with oil. It will probably be the cartridge type which is not serviceable. Purchase a new filter (take your old filter so the sale person can compare like for like). Before you refit the new filter, take out the sparkplug, it will probably be saturated in oil - wash it in petrol, let it dry then refit it. The engine should start and belch out great quantities of smoke. Don't be alarmed this will burn away. Once the smoke has cleared, check the oil level, fit the air filter and mow your lawn.

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.

Outdoor Power Equipment Parts Use the Part Finder Tool from MTD
Hope this helps

clutch slipery

Check your bladles , for turning freely.

if it is 2 cycle use 2cycle oil

maybe the yardman doesn't have an oil filter