Trash Cans.

You assume that the trash cans, being overfilled, will hold you up while you traverse the fence. You check if it can stand. You get up on one of the trash cans and try to hold the fence as you balance.

Unfortunately, the trash can could not hold you up thus making you fall back and hit your head on the concrete backyard. You're knocked out cold.

You now find out as you gain consciousness that it is morning and the landlord's dog is licking your face. Fortunately, your head does not hurt as much but unfortunately, you have failed to deliver the package.

MISSION FAILED