You would have uploaded and deployed packages to the app catalog site.
The apps deployed might contain errors, which will result in the invalid packages. In the screenshot given below, you can see the app, which is not valid, but there are no listed errors.
The invalid app will not be listed under contents page (add an app section) for installing. To identify the error, the apps for SharePoint list provides the extra field to list down the errors. In the apps for SharePoint list under app catalog site, modify the view, using library tab. Include App Package Error Message column in the view.
The screenshot given below shows the invalid app package with the error.
Thus, you have seen identifying the invalid package errors for SPFx solution packages.