Good job so far.

Here are some additional pointers

• GET /posts: right now you're returning HTML (with the div, h1, li, etc.). When you're doing a REST API, you will instead return raw JavaScript objects using JSON. The rendering with the HTML will be done client-side: it gets the raw data, and processes it to transform to HTML according the client-side application logic. You will explore the client-side part when doing the React module. In the meantime, you can just send the JavaScript objects from the server.
• GET /posts/:postId : looks good. Same remark as above: just send the JavaScript object directly.
• GET /posts/:postId/delete : you will have to remove the post from the array. One way to do this is to create a new array without the deleted post and change the value of the posts variable (you might have to change const posts = ... to let posts = ... in the code to do this). You should look at the array methods which might let you do this (see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array).