The problem is probably caused by an invalid package name "(not required)".
While I cannot explain how it got there in the first place (perhaps you're using an older version of the platform ...), changing it to a valid one, such as "com.mycompany.guestbook" should resolve the issue.
Note that expected behavior is for the package name to be generated automatically based on the application name + the preference setting Default Package Prefix for generated Java classes (in Window>Preferences>Tersus>Application Export Wizard).
Please make sure you're using an updated version of the studio (currently 2.0.18), as mobile support in general, and Android export in particular, have undergone significant changes (and bug fixes) since the last stable version (18.104.22.168).
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