Mac Computer

Without knowing your hardware version it's impossible to say.

You can use the table at https://everymac.com/systems/by_year/macs-released-in-2009.html to find your model, and it will tell you the latest OS that it supports.

Assuming it's an early 2009 iMac, El Capitan 10.11.6 is the latest version you can run.

Just to add, any early 2009 Mac can only run El Capitan.