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Microsoft Store has stopped accepting new app submissions with Windows Phone 8.x or earlier or Windows 8/8.1 packages (XAP or APPX). More information here: Important dates regarding apps with Windows Phone 8.x and earlier and Windows 8/8.1 packages submitted to Microsoft Store.
If you want to upload your project to the store, please port the Windows 8.1 package to a Windows 10 package.
Thank you.
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