Endpoint Insert Order requires billing information and eventually delivery address to be provided according to mandatory fields settings for each eshop (see in administration, under „Settings > Customers > Mandatory fields“ or using endpoint Required fields).
This is intended for new orders, however we have decided to allow relaxing these checks for historical orders (e. g. import from another system, which might be lacking some fields). To skip the check, use parameter ?suppressHistoricalMandatoryFields=true. This can be used only for historical orders (older than 1 day) and should be used carefully – other addons and functionality can be confused by inconsistent data and provide unreliable results.