Redux state is not updating after adding a blog post

I'd change the structure a bit:

in the createPost method instead of dispatching I'd just return data

export const createPost = ( postData ) => async ( dispatch ) => {
    try {
        const config = {
            headers: {
                'Content-Type': 'application/json'
            }
        }

        const { data } = await axios.post( '/api/post', postData, config );

        return data;

    } catch ( error ) {
        console.log( error )        
    }
}

and then in your code I'd

const submitHandler = ( e ) => {
    e.preventDefault();
    createPost({post}).then(data=>{
    dispatch({type:CREATE_POST, data})
 }

this should definitely work

The problem seems to be that React doesn't know that a new post was created. You'll need to call getPosts() after createPost() has been called.

const submitHandler = (e) => {
  e.preventDefault();
  dispatch(createPost({ post }));
  dispatch(getPosts());
}

If you want to structure your services in a more centralized, scalable way, check out my blog post on React services: https://schneider-lukas.com/blog/react-connect-rest-api. You could just as well store the posts in the Provider component to make them available to all components in the tree, or better yet create a separate Context, Provider, and Consumer for handling posts.