"Bad request: The data we received didn't match the format we expected for the updateJourney operation. Confirm that the information in your request is formatted properly..."

I have created a very simple Pinpoint journey, but I can't test it. The error provided is: "Bad request: The data we received didn't match the format we expected for the updateJourney operation. Confirm that the information in your request is formatted properly, and then submit your request again."

I don't know how to interpret this error. What does "the data we received" refer to? What's causing this and how do I fix it?