Hi to anyone able to help with this / having the same problem...
I use a 2009 iMac A1224 20" with a SSD, running macOS El Capitan (the shop which installed the SSD also installed El Capitan back in 2018.)
I recently been having some problems - spinning beach-ball, white screen / load bar freezing at 50% during start up.
I cleared out disk space, installed updates, reset PRAM and verified the drive - none of this seemed to help.
Luckily it does actually start up every 1/20 times, so I've backed everything up externally.
Initially I tried to re-install macOS, entering recovery mode using Command + R or Command + Option + R but only got in with Command + Option + Shift + R - I don't know if that's because it is an old model?
When trying to re-install El Capitan I keep getting "This item is Temporarily unavailable" with my Apple ID. I've tried two old Apple IDs and had not luck either. I've logging in to my new/current iCloud and Apple ID (made after these problems started happening) and seen my iMac is listed under my devices.
I recently tried erasing the SSD / Macintosh HD but it's not made any difference. Now when I start up I loads straight into Recovery Mode automatically.
Also since erasing my SSD, the newly named partitions do not appear in the Recovery Mode Startup Manager - the section 'Choose Startup Disk: Select The system you want to use' is now blank.
I am stuck in recovery mode and unable to install macOS X El Capitan.