boot disk not existing

Try booting into Internet Recovery Mode using Command + Option + R to access a later macOS online installer such as Monterey or Ventura (probably will be Ventura). If you boot to the online macOS Monterey or Ventura online installer, then make sure to use Disk Utility to erase the whole physical SSD as GUID partition and APFS (top option) or the install will fail.

If you still can only boot into the High Sierra online installer, then maybe try changing the system date to 2020 to see if it makes any difference in case the certificate for the High Sierra installer is expired. I've seen a few other recent posts where people have had trouble with the online High Sierra installer, but no one has followed up whether changing the date solved this problem. Unfortunately Apple has several different error messages for various installers when an expired certificate is encountered.

Do you have access to another Mac from Late-2009 to 2023? If so, then you can create & use a bootable macOS 10.13+ USB installer instead.....the installer you can create depends on the model of the other Mac since it must be compatible with the OS you want to create an installer for.

Create a bootable installer for macOS - Apple Support

Yes, High Sierra and higher operating systems depend on APFS formatting which is not available to Sierra and older operating systems. If you are trying to downgrade, you either need a separate HFS Extended GUID partition or external drive, or to completely wipe the newer operating system from the drive as an HFS Extended GUID partition, assuming you have backed up a bootable newer operating system to an external drive.

Test that the external drive is bootable before you do anything to the internal drive, by holding the Option key at startup and see if the external drive shows up, is selectable, and boots. Apple menu -> About This Mac will tell you which drive is the boot drive.

You can see Sierra partitions from High Sierra and higher, but not vice versa.

Edit. Platter drives are the only ones not automatically reformatted APFS upon upgrading to a newer OS unless it is Mojave or higher. Although you can force change the format with Disk Utility, keeping in mind that the format wipes the drive or partition selected.