There are 42 states where you can register to vote online, these are the ones where you can't:

Arkansas
Mississippi
Montana
New Hampshire
North Dakota
South Dakota
Texas
Wyoming

If you've seen websites suggesting that you can register online in those states, that is a voter suppression technique, specifically targeting younger voters who are much more likely to vote progressively. If you live in any of these states, make sure you can vote, it usually must be done by mail or in person.