So I just signed up for an account to see if I could successfully make a request, and had no problems doing so with this workflow:
You might be using an older version of the HTTP Request node - Photoroom’s API suggests sending the binary data in the image_file field, which your version of the node won’t let you specify.
In a more recent version of the node, you can specify the Form Data, like so:
Which then gives you a successful request 
Can you try either using the workflow I posted as a base, or maybe remove the current HTTP node you have and re-add it through the node menu (instead of copying and pasting from an older workflow)? That might fix this up!