It is likely MV3 Series worker isn't available in your current WebSpace. As a test, please try to create a new App Service Plan, in a new resource group, in the same Azure region as your current plan. To do this you can open a cloud shell and run command similar to below (modify name, resource group, location, etc., it to fit your needs):
az appservice plan create --name P1mv3plan --resource-group new-resource-group --location westus --sku P1MV3
If above succeeds, please navigate to the new App Service Plan you created and verify pricing plan is P1mv3. What you can do is re-create your current web app under this plan, and when finished and you have tested that everything works, delete your old app service plan and app. You will need to make necessary changes like modify your DNS records, add custom domain, certificate, etc.
Unfortunately you cannot switch your current web app to the new app service plan since the new plan has to be in a different resource group.
Please click Accept Answer if the above is useful.