I assume you've had users open the app at least once before trying to install the widget? From the Apple docs, For an app’s widget to appear in the widget gallery, the user must launch the app that contains the widget at least once after the app is installed. https://developer.apple.com/documentation/widgetkit/creating-a-widget-extension

Once I got to https://developer.apple.com/account I had to change the organization associated with me in the top right corner dropdown.