In your next PM, give the artist a specific amount of time to respond to you (for example, 1 week). If the artist does not respond within that time frame,
file a scamming report.
Do not cancel the trade. This is because you have already been given some art, even if it isn't exactly what you were expecting.
The artist and you will either come to an agreement or the moderator investigating the scam report (should it come to that) will give your instructions on how to move forward.
If you would like help setting up the PM to send to the artist or have any other questions, feel free to PM me.