This can also be caused by having fields with values not accepted by Shopify. For instance, only certain values may be entered for the Fulfillment Status and if you manually enter an incorrect value, this will cause an error.
Lastly, this can often be caused by contact/customer information that is invalid. Most commonly this happens with email addresses. Shopify validates all contact information before importing and will reject fake or incorrect values. So if someone entered an obviously fake email address, Shopify will reject this order and Orderimp will show an error. This can be resolved by manually entering a fake (but valid) email address such as test@example.com.
Feel free to reach out to support@orderimp.com if you need any additional assistance!