3 possible reasons:
1- Your Solution Configuration is on Debugging , and you must select Release for deployment.
2- Uncheck the Use total due date box. When this is selected, and you are in release mode, and the export option will not be available at all.

3- You have deployed the previuos version and have not yet modified the Android Manifiest file. To change the Droid project, open the properties and go to the Android Manifiest tab:
Version Number and Version Name .

The same values of the previous deployment?
Try changing these values.
Xamarin seems to disable this option until you change it to the new values.
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