Need Assistance with Enabling Permanent File Deletion in Scoped Dropbox App

SandeepCNT Здравко is correct; the files.permanent_delete scope can only be used by team-linked apps, and so can only be authorized by team admins. If you try to authorize any team scopes while signed in to a non-team account, you'll be prompted to sign in to a teams account instead. If you have a teams account, make sure you're signed in to that when attempting to authorize this. Otherwise, this functionality won't be available.