Need latest OS for iMac 27-inch mid 2011.

I have an iMac 27-inch mid 2011. It is a 2.7GHz Intel Core i5. I am on Sierra 10.12.6. I cannot seem to upgrade to a newer version of OS? Can anyone tell me what the latest supported OS is for this computer?

I have multiple apps that are telling me they're only supported on Sierra 10.13 or later. But I can't seem to find anywhere I can download Sierra 10.13.

Any help would be greatly appreciated.