To start, check the Azure stack marketplace for the image with the exact publisher, offer, SKU, and version that AKS is looking for:
Publisher: microsoft-aks,
Offer: aks,
SKU: aks-engine-ubuntu-1604-202007,
Version: 2020.09.14
If you are not able to find it, it is possible that you downloaded the wrong image. If you do not see your Ubuntu with that exact SKU and version, try to download the exact one from the azure marketplace. If you do see that exact parameters in your Azure Stack marketplace, let me know!