Even if you downloaded macOS Sierra from the App Store using your current Apple ID, you may not be able to obtain it now. Apple has not only withdrawn it from its list of products available in the App Store, but has removed it from the Purchased tab of the App Store app.
According to Apple, there are only two ways of obtaining its recent operating system installers: from the App Store, or when started up in one of the variants of Recovery mode.
If you have previously downloaded El Capitan from the App Store, it remains in your list of ‘purchases’, and you can always download the installer again through that account, but only that account. If you no longer have access to that account, then it is gone forever.
This is not true of Sierra: if you previously downloaded the Sierra installer from the App Store as a ‘purchase’, you will discover that Apple has now removed it from the list shown in the Purchased tab of the App Store app.
If you are using a Mac which has not been upgraded to Sierra 10.12.4 or later, there are only two variants of Recovery mode available to you:
- local recovery (Command-R), which will install the latest version which was already installed, and cannot upgrade macOS;
- remote recovery (Command-Option-R), which will install the version of macOS or OS X which came with your Mac, and cannot upgrade macOS.
Indeed, for those Macs which have been upgraded to Sierra 10.12.4 or later, the new variants will only install the latest compatible macOS, which is High Sierra, or the version which came with your Mac.
So if you are still running El Capitan, you cannot now upgrade to Sierra, only to High Sierra, unless you happened to download the Sierra installer and kept a copy. You might like to speak to a friend who still has a copy of the Sierra installer, though, and try that. I should point out that that would be a violation of its licence terms, though.
You may wish to speculate as to why Apple has done this with Sierra, but has left El Capitan in the Purchased list. Some Macs cannot be upgraded beyond El Capitan, although in theory all Macs that can run Sierra can also run High Sierra. But there are many users who rely on products, including some of Apple’s, which are compatible with Sierra but incompatible with High Sierra – Apple’s older Pro apps are an example. Is Apple using this to try to force users to upgrade to High Sierra, knowing that for many it is a less than attractive upgrade?
Whatever Apple’s reasons, the complete withdrawal of Sierra only a year after it was first released is hardly supporting users, is it?
(Rewritten 1000 28 September 2017 to reflect changes in App Store policy.)