It's an old post but since nobody responded I add my own vote. I was wondering too. According to this post... https://forum.adalo.com/t/configuration-api-photo-room/17599/2 ... it says, integrating is not going to work without a service like Integromat. Probably the answer is the API requires the image in Binary format so that is possibly why you are getting the error.
Unfortunately I don't have skills (and patience likely) to try configure the Integtomat API module because it's not like the other modules, so it does require to have some idea about how API works.
Zapier now has integration to Photoroom, so if you are looking for one it may work. I don't use Zapier because of their cost and prefer Integromat as it's more powerful.
After looking at the Photoroom website further, their site requires a modern hardware, it doesn't run on any of my machines. Here is what it says..
Unsupported browser configuration
WebGL and Hardware Acceleration need to be activated for PhotoRoom to work.
The website works for the one-off generated background removal, but login into the site doesn't work I get bounced by the message.
Too bad this forum is hollow now and don't see many experienced users contributing answers.