The cactus frailea is becoming sunken. Also, every time I plant the pith from the avocado, it dies even with a big root.
Cactus Frailea and Avocado Pear Problems
Certified GKH Gardening Expert
Your cactus has likely suffered from root rot.
Make sure you are growing the cactus in a cactus planting medium and not over watering.
Here are some articles that will help you with growing Avocado.
Here is a link with more information.