This happens when you place a door *slightly* too far back/forward from the frame. Two ways to fix it.

Sell the door and replace it, taking care to line it up exactly with the gap (it took me a bit of practice and I still occasionally screw up). Alternatively, use the same flooring type from your room to cover the gap.

Once you've fitted lots of doors you'll get to know exactly where they need to be placed to avoid these gaps :)