When editor does not respond to the request for withdrawal
what should I do?
Send another email stating that you will consider the article retracted if you do not receive a response within one week.
Can I submit the manuscript to another journal?
Yes, after one week.
Responses to comments:
I would likely do this but I am not convinced that it precludes troubles. Imagine the same referee getting it twice. It would be confusing at least. Just to mention one case.
Referees regularly receive manuscripts twice, for instance, when a manuscript is rejected. Admittedly, this situation is different, since the reviewer may not have been notified of rejection. However, such a reviewer will likely assume that the paper has been rejected. Even if they don't, they will likely contact the editor, who will be able to promptly deal with the situation. This could even be pre-empted, for example, an unacknowledged retraction could be explained in a covering letter.
[this] does not avoid...issues such as when the first journal and second journal both publish it...For all you know, the author may ALREADY have assigned copyright to the first journal.
From the question I assume copyright hasn't been assigned nor is the manuscript camera-ready.
Just to add to the answer from @user2768: when you submit the manuscript to another journal, mention in your cover letter what happened with the previous submission (including your one-sided correspondence with the editor). I would write this in as neutral a tone as possible; the point is not to hurl accusations at the editor of the first journal, but to avoid any appearance of deception on your part in the eyes of the second editor.
It's impossible to say in general that this will not be an obstacle to your second submission. As an associate editor myself, I believe it should not, but this kind of thing would definitely be looked at by the editor-in-chief.