Hi
One of causes for this message could be that you set minimum number of products in your Shipment Method settings.
Example: if Minimum number of products is set to 10 and someone had only 5 articles in Cart, checkout will throw message you mentioned.
QuickFix 1: In Virtuemart language file, change this message phrase into "Min. order quantity is XYZ"
Long term fix 2: Devs could expand Virtuemart messaging functionality to include dedicated message/s if delivery method limits aren't met.
Regards