A mystery! 🕵️
This will go a lot easier if you could provide the save file where the error occurs. 😊
On Aquilo P.U.M.P. should offer a tile selection like shown here:
https://github.com/Xcone/factorio_pump/blob/master/graphics/Menu.png?raw=true
Specifically the option below "Select cover for meltable tiles".
If this option doesn't show (as per your screenshot), then apparently P.U.M.P. doesn't think you need a cover tile. There might be an issue in detecting if meltable tiles apply? I've tried to keep that detection fairly generic, in case other people implement the meltable tiles mechanic on additional planets via mods. But I've never been able to test if that actually works.
Do you have other mods installed that disable the need for foundations on Aquilo, or that add additional planets that require foundations before being able to build, or anything else skirting that kind of topics?
With a save file I can just press "sync mods" and check this for myself.