Yes, you can cast on celebrities. They’re people like everyone else. However, one thing you should keep in mind is that a lot of people cast on celebrities, and throw at them everything from love/lust spells, to influence spells, to death curses (we had one guy come to this forum who believed Emma Watson was destined to be his wife, for example, because he had a dream about it, so he was throwing around every love spell he could find to try and make it happen).
Like politicians, celebrities thrive on attention (especially if they’re “internet celebrities,” who have no actual talent), and most already have dozens, maybe even hundreds, of people wanting to use their celebrity status to further themselves, so it can take an extraordinary effort to overcome that gluttony of energy.
In my opinion, a single petition isn’t going to cut it. This person probably has many people looking for their endorsement. However, I have read stories of people successfully using the Law of Assumption to get a celebrity to reply to them on social media, so that is probably your best bet. It takes the amount of energy being thrown at the target out of the equation entirely by assuming the goal is already done. It does take patience to wait for reality to catch up, though.