Since dovetail joints are difficult to mark and cut, certain errors such as gaps are easily made. To fix this, you don’t have to waste another piece of wood. Instead, get a saw and saw a thin piece of wood from a used wood that you have preferably with the same grain as the wood panel you are using. Once you cut the thin piece of wood, wedged it into the gap until it reached the bottom, and then cut the areas that stick out. Get a jack plane to plane the wood and then sand it to make the surface even and smooth.