Here's a small guide that should help you successfully complete the Bigfoot task:

1. Make sure that you put the dog in the Pet slot.
2. Have enough Pop Guns.
3. Bigfoots hide near special warning signs. Find them and then slowly walk around them with the dog.
4. If you still can't get a Bigfoot to appear, get a bomb and, without placing it on the ground, move your mouse across the map. If the bomb turns red, there is a Bigfoot hiding there. Do not use the bomb, just walk in this place with the dog.

❗ If this doesn't help, clear your cache, uninstall the game for 5 min and install it again. Don't worry, your progress won't be lost.