@JaghadishR: This is now solved - was a silly mistake but for others to note
While configuring the Service Package from which this API is run - the access group should have access to the properties defined - else this error would pop up - so careful in selecting the access group - cheers
I also had this error (Pega_API_055) using Postman. In my case, i had to set the access group of Operator ID used for basic authentication (in Postman) to the one that had access to the case type that i was trying to create using POST/cases API. That fixed it.