In the attache file, I have re-created both the sweep and the case bottom using different techniques and the results still won't 3D Add.
1: Quick Pull on the swept object's lower facets, but that didn't work, so I Extracted Face (2019 tool) with the desired thickness and 3D Added them to the swept object, but the result still did not 3D Add to the Case Bottom.
2: Save As in earlier formats (this is in 2017 format so that more people can try to fix it) for experimenting with different versions of TurboCAD, but still no joy.
3: Increased the Faceter Quality for each object (0.001" and 2°).
4: Changed the profile of the swept object a couple different ways, with no change in result.
5: 3D Added a Box to each object to see if I could find the defective object.
6: Split the Case Bottom two different ways to see if that would help.
Running out of ideas, here.
TC Pro Platinum 2020, 2019, 2018, 2017, 2016 & 2015 (all with LightWorks & RedSDK) & V21
System: i7-5820K @ 3.30GHz, ASRock X99 Extreme4, 16GB DDR4-2133 RAM, Gigabyte GTX 970, Samsung NVMe SSD 950 (256GB), Windows 7 Pro (64-bit) SP1