Sounds like the export worked, and you can’t find the file or share it from your tablet. All of that is delegated to the standard Android system apps and basically outside of the PractiScore app.
Emailing files from device is outside of the PractiScore app. You either need to install/configure some email app or if you have tablet’s manufacturer app, maybe you need to use some different email app instead. Or instead of emailing try to share through DropBox, Google Drive, OneDrive, Pushbullet or similar sharing services.
As for saving to the local tablet storage, it also depends how manufacturer set things up on device. If you want to save files to a specific location you can install “Send to SD card” app, it will be one of the sharing options then. See https://play.google.com/store/apps/details?id=ru.gelin.android.sendtosd