The Multiplay platform is engine agnostic and can be used to host gameservers made in most modern game engines, such as Unity, Unreal and game engines, and even versions which are a few years old.
However, in order to use the Multiplay SDKs you will need to use a supported version of the Unity engine which is currently any Unity version after 2020.3, or any version of the Unreal engine after version 4.22.
Note to Unity Engine Users
There is a known bug in Unity Engine 2022.1.x and 2022.2.x before 2022.2.3f1 where HTTP requests are not sent to the Multiplay API properly. If you are using one of these versions, you need to upgrade to using Unity version 2022.2.3f1 or later.
If you continue to have issues even after updating, please raise a ticket here.