This error VMExtensionProvisioningError generally means that some extension deployment got failed ! If you go to corresponding VMSS - you might see instances might be in Running state but the VMSS (overview) might show you that VMSS is in failed state. If you click on that failed status it will show you extension failure along with the detailed error message & error code.
Basically whenever the AKS VMSS nodes gets bootstrapped as a part of post deployment operation those nodes will try to reach out to mcr.microsoft.com or ubuntu.com to deploy additional softwares on those nodes. If those nodes don't have any outboundconnectivity to reach out to the internet , that's where in most of the cases you will see "Extension" failure error messages.
Probable issues can be:-
-> Check if you are using customDNS servers , if yes - check if those custom DNS servers have got resolvers to Azure Provided DNS or not
-> If you have any firewall , kindly check the logs if that is blocking
-> In the firewall , check if all the outbound rules have opened or required FQDN rules are opened based upon the below document: https://learn.microsoft.com/en-us/azure/aks/outbound-rules-control-egress
-> In the networking section , check for any blocking NSGs
-> If it is a private AKS cluster , kindly see the pre-requisites
Let us know if that helps !