Do you know why my new finished basement foundation wall option is not saving?

edit

I can reproduce the issue, it's a bug in BEopt that only seems to affect this category.

Unfortunately there is no easy way to workaround the issue -- the best advice is to pick the closest option (or do two multiple runs with different options and interpolate/extrapolate).

(The ceiling joist height is an input used in conjunction with the rim joist insulation.)

I'm not sure why it cannot be saved, but note that Continuous Insulation Nominal R-value is total R, not R/in, so your entry should be 20 not 5. The Whole Wall R-20 XPS is probably your best bet. As for ceiling spacing values, those are so that it can calculate how much of the rim joist is thermal bridging to the floor versus exposed/insulated.

BEopt's Slab entries are for slab-on grade. Since the ground's temperature below the frost-line/in a basement is relatively constant compared to the ambient temperature, the software simplifies inputs and ignores under-slab insulation. This is a fairly reasonable simplification unless you have radiant heating in the slab, which BEopt itself does not explicitly handle anyhow.

I just ran into this same issue. One work around is to use BEopt 2.7, this bug doesn't seem to be present in the older version of BEopt and I was able to model a custom basement wall. You can update the BEopt 2.7 file to 2.8, but when I attempted to add the option to the library, ran into the same error message.