Unfortunately, once your key request has been rejected, you cannot re-request the key again. But don't worry! Keymailer is continuously being updated with new games, both indie and mainstream, which you may apply to.
Keymailer is also conscious that circumstances might change and they are looking at other options in this scenario, such as a waiting list, for example.
It's highly recommended that you keep on improving your channel and creating content to increase your chances of being granted a key.