Your lawn-mower should have an oil-dip-stick with "minimum" & "maximum" oil-markings.
If it reads below the "minimum" oil-marking,
add a little oil, replace & tighten the dipstick-cap,
and crank the engine without starting it,
then check the oil-dip-stick again.
Do NOT exceed the "maximum" marking,
too-high oil-pressure can damage your-lawnmower.