Cannot read Time Machine backup

[edscdk]

I backed up the Mac (Big sur, late 2015 imac 21") , to a external HDD,

stuck in a SSD

rebooted using command R,

selected restore from time machine, and nothing...

Disk Utility says the disk is not initialised,

I assumed id messed up something, got a new hdd, did another backup, checked it looked ok, and the same happened...

What am i missing?

 

[edscdk]

answered my own question, thought id reinstall the OS then try the restore,

turns out the network boot of a Mac 2015 boots High sierra so obviously that cannot read the newer time machine backup, i guess ill have to download the iso or install the old version then upgrade