The Server SDK is targeted for your backend server and GameLift currently only offers support for Windows and Linux server types. If you are using the Unreal Server plugin on mac, then that would be an unsupported platform combination (EC2 Mac instances are not currently supported for servers in GameLift).
Its totally possible to build iOS clients that use GameLift that talk to your Windows/Linux based servers. But obviously your development path here is slightly harder and a lot depends if you can find a path to support your server builds (you can for example use Docker builds, EC2 instances etc to get Windows, Linux servers built if you don’t have a machine).
https://docs.aws.amazon.com/gamelift/latest/developerguide/gamelift-sdk-client-api.html