Sometimes you can end up with a gap between the cake and the board. There’s an easy way of filling it.
Prepare a piping bag. You'll find instructions how to make and fill a piping bag here.
Make softened paste from the same paste that you used to cover the cake so that the colour is exactly the same.
Fill the piping bag with the softened paste.
Squeeze some softened paste into the gap as shown in the photo.
Use a soft, damp (not wet) brush to remove the excess paste. Hold the brush upright as shown in the photo as if you were trying to clean the board from the excess paste. Don't push the brush into the gap.