How can I get my lawnmower bolt off?
Allowing for the inherent mechanical weaknesses of a ratchet wrench and the limited lever arm provided by such a short handle, consider to purchase or borrow a 15 mm box end wrench. The handle may not be much longer than the ratchet wrench, but will more safely accept a lever extension.

A strong steel pipe of a meter length or better, with an inside diameter to accept the box end wrench will give you an incredible lever arm.

If you are confident that you are turning the bolt in the correct direction and that it is designed to be removed in this manner, you are likely to free it up easily. If not, it's also possible to snap the head right off, or worse, break something internally.

Your local mower shop may also accept the mower and have the necessary tools to pull the blade. You may have to pay a bit more for the first time, but it's also likely it won't be as difficult to remove the next time.