Hey @Crystallia, thank you for your post! Happy to help you answer your question.
This error usually occurs when physical and non-physical items with different fulfillment types can't be purchased in the same order. You can get rid of this by setting the items so that they have the same fulfillment type (shipping, pick-up, delivery).
If you have done the above and the issue is still occurring, let me know! Would be happy to take a look further into the issue for you.
