Extremely hard layer after cooking the burger on the grill It was the first time I tried making burgers at home. I did some research on the subject. I just put salt and pepper into the ground beef.

Then I heated the iron grill well and applied a small amount of olive oil on the hamburger patties and baked each side for 7 minutes.

I share the photos of hamburger meatballs before and after cooking.

What do you think is causing this situation?

1- Minced meat is too fat?
2- Cooking time?
3- To put olive oil on it before cooking?
4- Iron Grill?

I will be grateful if you could help me. I don't want my next attempt to end like this.

Thank you.
You did everything right except you overcooked your burgers. Those are relatively thin patties, they won't take 7 minutes a side on high heat, that is what caused that hard crust. I'd be cooking them 3 minutes a side at most. 

A small thing but I'd suggest you replace olive oil with corn, sunflower or another high temperature oil. Olive oil will smoke at a high temperature and add off flavors. This had nothing to do with your thick crust.