Let me be a bore a little more ;) solidity ^0.8.10

posts[postCount] = Post(postCount, _content, 0, msg.sender);

to

posts[postCount] = Post(postCount, _content, 0, payable(msg.sender));

---

emit PostCreated(postCount, _content, 0, msg.sender);

to

emit PostCreated(postCount, _content, 0, payable(msg.sender));

---

address(_author).transfer(msg.value);

to

_author.transfer(msg.value);

---

Possible reentrancy vulnerabilities. Avoid state changes after transfer. I think you can do this:

     function tipPost(uint _id) public payable {

        //Ensure the id is not invalid
        require(_id > 0 && _id <= postCount, "Valid id required");

        //fetch post
         Post memory _post = posts[_id];
         address payable _author = _post.author; //fetch the author
         address(_author).transfer(msg.value); //transfer Ether to the author

        //We increment the tip amount
        _post.tipAmount += msg.value;

        posts[_id] = _post;

        // let's trigger a post tipped event
        emit PostTipped(_id, _post.content, _post.tipAmount, _author );

     }

to

function tipPost(uint256 _id) public payable {
    require(_id > 0 && _id <= postCount, "Valid id required");

    posts[_id].tipAmount += msg.value;
    posts[_id].author.transfer(msg.value);

    emit PostTipped(_id, posts[_id].content, posts[_id].tipAmount, posts[_id].author);
  }