There are a few things in those errors, while they sound the same, the actual causes are different:
InvalidOperationException: You are trying to read Input using the UnityEngine.Input class, but you have switched active Input handling to Input System package in Player Settings.
at UnityEngine.Internal.InputUnsafeUtility.GetButtonDown (System.String buttonName) [0x00000] in <00000000000000000000000000000000>:0
at UnityEngine.EventSystems.StandaloneInputModule.ShouldActivateModule () [0x00000] in <00000000000000000000000000000000>:0
at UnityEngine.EventSystems.EventSystem.Update () [0x00000] in <00000000000000000000000000000000>:0
InvalidOperationException: You are trying to read Input using the UnityEngine.Input class, but you have switched active Input handling to Input System package in Player Settings.
at MM.Update () [0x00000] in <00000000000000000000000000000000>:0
InvalidOperationException: You are trying to read Input using the UnityEngine.Input class, but you have switched active Input handling to Input System package in Player Settings.
at MainMenu.Update () [0x00000] in <00000000000000000000000000000000>:0
InvalidOperationException: You are trying to read Input using the UnityEngine.Input class, but you have switched active Input handling to Input System package in Player Settings.
at SoundManager.Update () [0x00000] in <00000000000000000000000000000000>:0
Look at the callstacks. The first one is coming from UnityEngine.EventSystems.StandaloneInputModule. This is a component that is assigned in your scene to the Event System object - it provides input for the UI. StandaloneInputModule is specifically the module that provides input using the old system system, which errors out if you switch to the new input system. The solution is to replace that component with InputSystemUIInputModule, which does the same thing except it sources the input events from the new input system. Note that these events are provided for uGUI, also known as Unity UI. IMGUI is an entirely different UI system and doesn’t seem to have anything to do with this issue.
The other errors are coming from your scripts: looks like you are calling old input yourself in classes like MM, MainMenu and SoundManager. If you want to upgrade to using the new input system, you will have to change your code to use the new input system.