These are the steps I follow, I have created a collection product and I have my primary and secondary subscription keys.

First step was to create an API user when was successful
 

Second step, i created an API key for the API user which was successful.
 

Third step, I created an access token for the user under the collections product subscription.
 

Fourth step, I initiated the requestToPay request but it keeps failing with a 400 Bad Request. Is this an issue currently with the system (going by the error, it is not) or is it specific to me?