Is this helper app nested within your main app?
No.
That means you selected your main app’s scheme and chose Product > Run, right? So you’re debugging the main app and hit this problem when the main app tries to run the helper app.
Correct.
Is any of this stuff sandboxed?
No.
If not, do you have the hardened runtime enabled?
Hardened runtime is enabled for the main app, not the helper.
Is the main app running the helper as a child process (fork then exec*, or posix_spawn, or NSTask, or Process, or similar). Or as an independent app (NSWorkspace, or Launch Services, or similar)?
Running as an independent app using -[NSWorkspace openApplicationAtURL:configuration:completionHandler:]. The configuration includes an additional argument.