What is the latest operating system I can use
I'm running macOS Monterey 12.7.6 on my 21.5" iMac late 2015
2.8 GHz - quad- core intel core i5
With 8GB 1867 MHz DDR3 memory
1Tb macintosh HD
iMac 21.5″, macOS 12.7
You are in the last compatible macOS Monterey for your 2015 MacBook , that Mac can’t be update beyond macOS Monterey