Getting error while registering reducers using ActionReducerMap: "not assignable to type 'ActionReducerMap<AppState, Action>"

Your error message says that property post has the wrong method signature. It is (state: State, action: Action) => void, but should be (state: State, action: Action) => State.

In post.reducers.ts your reducer needs to return the state that is passed into it like this:

export function postReducer(state = initialPostState, action:Action) { 
  return state;
};

The return type is being implicitly inferred from what you are, or in this case aren't, returning.

You could explicitly state the return type with ...): State {... but you should still return the state.