I resolved this error by setting the Deployment Info to the correct version of iOS (15.0) for the Widget Extension - it was previously set to 16.0 which the widget and app don't support.
Took a while to figure it out. There might be multiple causes for this error and one expects a better description from XCode to help developers identify the issue.
azk08
Users receive reputation points by contributing quality content to the forums.
Posts include post and replies published on the forums.
Post authors can mark replies as Accepted to indicate successful solutions.
Apple Developer Forums admins can mark replies as Apple Recommended to indicate an approved solution