Open GL not available fixed? iMac 27", Mid 2011

Hello, we have downloaded the Launcher but when we proceeded to try to open Fortnight we got this message.

OpenGL Not Supported
You must have a a Metal compatible graphics card and be running Mac OS X 10.11.6 or later to launch this process.

We are running Mac OS High Sierra Version 10.13.1
iMac (27-inch, Mid 2011)
Processor: 2.7 GHz Intel Core i5
Memory 32 GB 1333 MHz DDR3
Graphics: AMD Radeon HD 6770M 512 MB

Please let us know if this is even possible. I’ve read a few threads but no one seems to have a definitive answer.

Take care