I tried to dual boot Mac OS X and macOS but it won't boot off of the macOS partition anymore. HELP!

I was installing OS X Mountain Lion alongside macOS Sonoma but now I can't boot into the Sonoma partition. I would be so thankful if someone could help me fix this. Thanks! (Note: Using a Late 2012 iMac (13,2), I want to keep my data on the Sonoma partition if possible)