Error Blueprint Runtime Error: Accessed None trying to read property K2Node_DynamicCast_AsMy_Game_Instance from function:

Error Blueprint Runtime Error: Accessed None trying to read property K2Node_DynamicCast_AsMy_Game_Instance from function: ‘ExecuteUbergraph_MainMenuMap’ from node: Show Main Menu in graph: EventGraph in object: MainMenuMap with description: Accessed None trying to read property K2Node_DynamicCast_AsMy_Game_Instance

Looks like your level is finished constructing and fires Beginplay before any Gameinstance does.
what happens if you put a tiny delay before the Cast node?

I go to the project settings and set game instance to my game instance.Problem solved,Thanks