When macOS High Sierra releases, expected to be September 25th, 2017, macOS Sierra will no longer be available on the App Store. We recommend, that unless you are planning to skip Sierra, that you download macOS Sierra from the App Store before September 25th, 2017 even if you don't intend to install it right away.
It will then appear in your Purchased tab in the App Store so will be able to access it and download it in the future but if you don't get it before macOS Sierra is released then it won't be available from the App Store.
Once you have downloaded the installer app, it should ask you to continue the installation. But you don't need to do the installation there and then. Instead, quit the installer app at this point and you will have the Installer in your Applications folder ready when you want to install Sierra.
Planning To Install macOS Sierra?
If you are planning to install macOS Sierra then do check out our Mac OS 10.12 Sierra - Pro Audio Compatibility Update article where you can see all the 10.12 pro audio compatibility information, to check all will be well with your set up before you update it and break something that isn't compatible with macOS Sierra.
Should I Install macOS High Sierra When It Is Released?
Our current advice is absolutely not. As we saw with macOS Sierra most pro audio apps were not compatible with Sierra for a while, some for quite a while. We will be producing a macOS High Sierra Pro Audio Compatibility Update article as soon as we have some initial information to share with you and then, as we did with the Sierra article, we will update the High Sierra article as information becomes available.