Is it obligatory to advise everyone who errs, especially upon seeing mistakes in comments on websites, and what is the ruling if advice is not given in the event of not being registered on the website?

Enjoining what is good and forbidding what is evil is a communal obligation (fard kifaya), and it is performed according to one's ability. If you see an evil, change it with your hand; if you are unable, then with your tongue; and if you are unable, then with your heart.

If you are able to correct an error and there is no one else to do so, then it is obligatory upon you to respond to violations of Islamic law. This should be done according to your ability, while observing knowledge, gentleness, and forbearance.