Wiped drive, now can’t reinstall El Capitan from Recovery because the "item is temporarily unavailable"
I ran into this issue myself.  NoahL's comment is spot on: try Internet Recovery (Command+Option+R).

I was trying to reinstall macOS on a laptop.  The on-disk recovery kept giving me this error message after I gave it my AppleID in the reinstall wizard:

This item is temporarily unavailable.
Try again later.


I retried two weeks later, and still got the same error.
I found this thread and tried out Internet Recovery and I was able to reinstall macOS.



Note: I had to erase the disk with Disk Utility before Internet Recovery would let me reinstall macOS, because the version on disk was newer than the version that will be installed.

(I don't know why Internet Recovery doesn't just offer the latest macOS...)