How to view Failing Authentication Logs for a specific iOS Application
If you do not see the log file you need along the crash logs in XCode or Apple Configurator 2, then the log file is not available here.

In that case the log file is something proprietary to the app - i.e. for example just a text file that the app writes to during execution. In that case, the text file is within the app's document folder. You can use the "Files" app on the iOS device to look inside the app's folder for the log file.

If you do not see the log file there, I think you might be mistaken that there should be a log file at all. If you haven't got a manual, support web site or similar stating that there should be a log file, there usually isn't a log file.

You could use a program like iDevice Manager or similar to browse the file system on the iOS device to look for a log file within the app's folder. This is normally not something you want or need to do.