Am quitting without another job due to toxic environment and high work stress. What would be be a good reason to tell my boss when I resign?

Don't burn bridges you may have to cross later.

Read that again.

Then, be as tactful as possible when you take your leave.

Remember, if your reason for resignation is because of the toxic environment, don't allow room for temptations (be it extra benefits or a higher salary).

If the root problem isn't solved, the situation won't better itself.

Oh no, that's a difficult situation. It shouldn't hurt to be honest yet tactful, especially if you've already decided to move on. Consider telling your boss you feel as though you don't fit the culture of the company, or that you need a break from your current environment. Hope this helps!