Trying to use Replace Text

I'm not a coder by any means but I'm trying to create an automated business process. I have the front end done, where a user uploads two documents, and the solution is to see the the difference on the numbers in a table contained on page 2 and update them. 

I created a solution using Python, but it would create an image rather than updating the individual field. This is a no goal for a legal document, as I need the actual field, updated similarly to how you would through Adobe Acrobat. 

Looking over some documentation and through ChatGPT, it seems Adobe API should be able to replace numbers on document one with the right ones from document two.

I am a registered customer using PDF Services API with the DCAPI scope enabled. I can authenticate and use other endpoints (like Combine PDF, OCR, etc.) successfully as well. When I try, PDF Services API /operation/replace-text returns 415 Unsupported Media Type. 

• My credentials are valid and recently used.
• I have tried both via code and curl, same result.