Can you type with only one hand on a flip phone ?

Do you actually mean "flip"? Because when open, with the screen opened upwards, they have roughly the same dimensions as any modern slab phone. Then it'd come down to whether your hand is big enough to type one-handed on that formfactor (a few women, some men).

If you mean "fold", then probably only while closed, subject to the same hand-size limits. While open, with the screen opened to the left, like a book, they're quite a bit wider than even the largest human hand could reasonably type on.

Sure, just fold it.