MacPractice is currently in the process of ensuring we're compatible with High Sierra. However, you may be in a position where you need to update from an older macOS, and the Apple Store only has macOS High Sierra available. Thankfully, Apple has provided us a link that will allow you to download macOS Sierra.
As a reminder, if you have any issues with upgrading your macOS, you should contact Apple Support. This link is provided by Apple Support, and as such, we cannot guarantee that it will be available forever.