It's not illegal to use white out on a check you've written but most banks will not accept a check from someone that has white out used on it.
If you make a mistake while writing out a check then void that check and destroy that check and write out a completely new check without the mistake so that the bank will accept it.
Even if you cross out something on a check and write over it the bank still may refuse to cash or deposit the check for the person you wrote the check too.
So always just write out a new check and void the other one to make sure the check can be cashed.