Hi @aomololu01,
Thank you for your question. I could spot a typo in your payload; type: Role in your payload should be ROLE instead. But if that was the issue you would have got a 400 series error with invalid type in text.
I successfully replicated your error and found the text with 400 error you received is because of invalid ID provided, possibly: assignment ID: -
You do not need to provide assignment ID, perhaps change your payload to hold only the required fields as per create-access-request | SailPoint Developer Community.
Good luck 