Most commercial DVDs are protected with the Content Scramble System (CSS). This is to prevent them from being illegally copied. CSS is a Digital Right Management (DRM) and encryption system, utilizing a proprietary 40-bit stream cipher algorithm.
Wondershare UniConverter for Mac (originally Wondershare Video Converter Ultimate for Mac) is able to convert videos and rip most DVDs except Commercial DVDs. However, you can try third-party software like the Free DVD Protection Removal Plugin, which enables the product to rip both common and commercial DVD movies. Check the guide for its installation steps.