This HTTP code means Content Too Large response status code indicates that the request entity is larger than the limits defined by server.
And, from the Printify document,
413 Request Entity Too Large The request exceeded the maximum allowed payload size. You shouldn’t encounter this under normal use.
So, For this issue, you need to check the filesize that you are passing and cross-check it against the permissible file size allowed by Printify.
For this, though it is more on the Printify end, and what you can do is crosscheck the data you are sending and get in touch with printify regarding the issue, maybe with the payload that you are sending.