Insufficient thickness of the outer wall: Increasing the thickness of the outer wall of the print can effectively increase the strength of the foundation. Since the inside of the print is partially empty, the thickness of the outer wall can have a significant effect. Adjust this setting via the Wall Thickness parameter of the Basics page. For example, originally using a wall thickness of 0.8mm, try to use a 1.6mm wall thickness to see if the gap disappears. Insufficient solid layer at the top: Another common cause of weak foundation is the insufficient number of solid layers on the top of the print. A thin roof is not sufficient to support the structure printed on it. First make sure the "Top solid fill" option in the "Expert Settings" is checked. Then adjust this setting via the Bottom/Top Thickness of the Basics page. If you were using a top thickness of 0.4mm, try printing a 0.8mm solid layer on the same print to see if the base is up. The fill rate is too low: Finally you have to check your print fill rate, set in the Fill Density section of the Basics page. The top solid layer is built on the fill, so it's important to have enough fill to support these layers. For example, if the original setting is to use a 20% fill rate, try increasing it to 40% to see if the print quality is improved.