Writes can be complicated, which is why we store records of all the requests Merge makes on behalf of its customers in our Logs page! In the event you receive an error when you're posting a candidate or other object, you can leverage this to determine what exactly you need to do to correctly post.
You'll just need to navigate to the Logs page and filter for requests that have failed. The list will sort starting with the most recent call and descend from there. If there's a particular range of time you need to examine, you can also add that filter!
From there, you'll need to click into the relevant request to the API, and you can see exactly what error message the API returned!