Perhaps my most obscure suggestion...
You can't create a build spec with the same name as an existing one regardless of capitalization:

But if you use a mismatched case in "<vi.lib>\AppBuilder\AB_API_Simple\Build (path).vi" you get this error:

So either you should be allowed to create build specs with the same letters and different capitalization, or the API should be smart enough to find the matching build spec regardless of capitalization.
I filed Bug 2965316 about this inconsistency.