Apple recently announced their new macOS called macOS Sonoma. Apple has dropped support for many older Mac in macOS Sonoma.
Here is the list of devices that are officially supported by macOS Sonoma :
iMac 2019 and later
Mac Pro 2019 and later
iMac Pro 2017
Mac Studio 2022 and later
MacBook Air 2018 and later
Mac mini 2018 and later
MacBook Pro 2018 and later
If you want to install macOS Sonoma on older Apple computers you have to use OpenCore Legacy Patcher.
Direct download link for macOS Sonoma beta public beta installer build 23A5286i:
macOS Sonoma Public Beta 1