Lots of people can't squat with their heels on the floor. It's all to do with flexibility. I go to yoga and the age range is from 20-60. About half can get their heels on the floor and half can't. Those that can are most likely to be very young or have done yoga for the longest (it makes you more flexible).
If you watch your video and compare the angle that your foot and leg make when you pull your toes up towards you to those of the people sqatting on the video, yours is likely to not be as sharp as theirs. If so, the Achilles tendon at the back of your leg is probably a bit shorter.
You may also have less flexibility in other parts of your body making it difficult.
The way to successfully stretch your body has to be gradual. If you try to force yourself into a position you may hurt yourself.
Your Achilles tendon should be treated with special care as damage can cause great pain and take forever to mend.
If it causes you problems in normal life, I would recommend going to a physio. Otherwise, yoga is great...
If it's any consolation, people who naturally have overly flexible bodies tend more easily to sustain damage, especially as they get older. And there's not a lot they can do to help this. Those who are overly taught can always become more flexible with stretches and relaxation.