Why isn't new (erased/formatted) SSD showing anymore in Disk Utility (and can't install OS on it)? I just replaced the SSD in my MacBook Air (early 2015). I erased/formatted it in the Disk Utility (choosing APFS as desired format and GUID partition scheme) and then tried to restore my Time Machine back-up. Looked like it was going well and gave me circa 1 1/2 hours until all restored. Countdown continued. Fine.

But then instead I got prohibitory (circle with bar through it) symbol. Reading round, maybe too late an OS for my Mac? Or some other issue?

But when I try reinstalling the version previously on my Mac (using command + R on start-up), it gives me the option to agree to installing El Capitan, but I now can’t see the new SSD to install it to. Likewise, when I go into Disk Utility, I can’t see any internal drive anymore although it was showing up before I formatted it. (And I can’t see an option to ‘View > Show all devices’ referenced in various other threads.)

Any help (and STEP-BY-STEP INSTRUCTIONS!) appreciated. Just want to get my old stuff onto a larger drive and get on with my work!

Thank you.
I think you bought a NVMe SSD, did you ? What model is your new SSD ?
macOS High Sierra was the first to support NVMe SSDs, that would explain why El Capitan can't see yours.

You may want to try cmd-alt-R at boot in order to boot the latest macOS Recovery available for your Mac from the internet. It should be Catalina, which supports NVMe SSDs.