Hello, good day and thanks for your question. In general, Minecoin is non-refundable which still depends on the scenario and the circumstance that happened and in the event that a purchased world is not working at all you will need to contact or reach out to the community who developed that world to provide feedback and discuss any known issues or bugs. You can also check the link below for reference.
https://feedback.minecraft.net/hc/en-us/articles/360004166751-Marketplace-FAQ