Yea. there needs tobe something solid under the crank arm .

with a hole to let the cotter pin head fall through it ..

If really at an impasse your friendly LBS is there to help. bring it by..

old guy skill, hand filing the taper on the cotter pin , so it fits
without more than a slight, tapping from a hammer to re set it..
then the nut holds it .. .. you don't pull a cotter pin into place with a wrench ..