It looks like the JSON that is being sent is not being correctly converted, I did a quick test by sending the data to webhook.site but there are other tools to do this. To fix the issue try changing your Body in the HTTP Request node to…
Btw. if you have trouble getting it to work. There’s an issue with the sample data.
The notion API won’t create two separate file links if the urls are identical.
In this case it will only create the first one.