The new Xamarin Forms Shared Project Template has a bug which causes the Windows Phone build to fail out of the box. You might see an error like:

The 'ProductID' attribute is invalid - The value 'b79569cb-1898-4dab-9173-afe40fa8559c' is invalid according to its datatype 'http://WPCommontypes:ST_Guid' - The Pattern constraint failed.

Note that the error message you receive might be a little different depending on what value is not defined properly. You are in luck though, as there is a simple fix for this.

You can fix the issue by modifying the WMAppManifest.xml file in your Windows Phone project. You need to add curly braces (are there any other style?) around the GUID values for the ProductID and PublisherID attributes. In some cases there might be other GUID values that are missing the curly braces too so add them there as well.

And that's it! This should resolve your error and get you on your path to a successfully building application :)

Cheers,
Paras Wadehra
Microsoft MVP - Windows Platform Development
http://twitter.com/ParasWadehra
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My WP Apps

Via: http://paraswadehra.blogspot.com/2014/08/xamarin-forms-shared-project-template-error.html