why can i not install the latest macOS

See here: link-> macOS Monterey is compatible with these computers - Apple Support

If its actually a 2015 model, then it is compatible with Monterey.

If its a 2014 model, that was bought in 2015, then it is not compatible.

Verify your specific model in Apple Menu->About this Mac.

Tips Big Sir Upgrade the one just before Monterey

Tips Monterey Upgrade  the latest macOS - ALERT: If the computer has an After-Market internal drive - there are reports Monterey may not install. That is because of a EFI Boot ROM required to flash the computer. The BOOT ROM is expecting the Original Apple HD / SSD and if it is not there - it may Balk at Installing.

Check here to see if your Mac is compatible: macOS Monterey is compatible with these computers - Apple Support