Pages, keynote, numbers, garage band apps disappeared

All of those apps have never been included with macOS/OS X & need to be downloaded from the Mac App Store. Make sure you are signed into the MAS with your Apple ID. I have found that it may appear that you are signed in but, as you discovered, you can't download anything. I usually get the "you need to sign in" error even though it says I'm signed in. There is obviously some disconnect in the App Store. Click on "Store" in the menu bar & choose "Sign Out." Then go back & sign in.

Actually, vkvee and Peggy, while I, too, have often noticed that the Mac App Store (MAS) often appears as if I’m signed in, but not, I have found that simply choosing Store -> View My Account is sufficient to be presented with the login dialogue, so I can sign in and work properly within the MAS.