Error creating form: Ancestor for 'TFormName' not found.(Of course, it actually said the name of my form, not 'TFormName'.)
And when this happens, my .dfm (Delphi Form) won't load, so I can't do anything with the design of that particular form. To work around this, I had to open the ancestor (or in at least one case, multiple ancestors), and leave them open in the IDE, before being able to view my form's .dfm at all.
So I double-checked all my include and lib paths, as well as my include statements in the headers of my .h files. I googled my fingers to the bone, looking for any answer I could find. Nothing.
Finally, I noticed that the form I was trying to inherit from wasn't even in the inheritable items list, which led me to this old post, and I knew I'd found the culprit.
In the end, the solution was spectacularly simple: Remove all problematic units from the project. Then, add them back in the appropriate order. No more error!
It really was as simple as this: