I’ve been trying to use some of the newer features to me. Specifically the “solid tools”: trim, subtract, union, etc. I keep running into issues where I keep seeing “One or more of your objects is not a solid”.
So I made a brand new file and did a test. Both of the objects in the file are made from the same object. The only difference is that the object on the right has a 1/4" rounded edge using the “follow me” tool and is now no longer a “solid”.
By the way, I notice incorrect tag usage in your file. Make sure you keep Untagged active at all times and that all edges and faces remain untagged. Only the groups/components in your model should get tags.
Please complete your forum profile with SketchUp version, operating system, and graphics card model. That info helps us help you by letting us know what tools and capabilities you have with SketchUp.
To help achieve your goals, some time spent at the SketchUp Campus and at the SketchUp - YouTube channel will be very worthwhile. Both sites are from the SketchUp team. On the YouTube channel, pay attention to the Square One Series. It covers the basics for each tool.