This still seems to be a problem on a lot of regions, and not limited to bootstrapping

Deploying to e.g. azure/ukwest works fine -

   juju bootstrap azure/ukwest cdk1
   Creating Juju controller "cdk1" on azure/ukwest
   Looking for packaged Juju agent version 2.7.4 for amd64
   Launching controller instance(s) on azure/ukwest...
    - machine-0 (arch=amd64 mem=3.5G cores=1)
   Installing Juju agent on bootstrap instance
   Fetching Juju GUI 2.15.0
   Waiting for address
   Attempting to connect to 192.168.16.4:22
   Attempting to connect to 51.11.120.37:22
   Connected to 51.11.120.37
   Running machine configuration script...
   Bootstrap agent now started
   Contacting Juju controller at 192.168.16.4 to verify accessibility...
   
   Bootstrap complete, controller "cdk1" is now available
   Controller machines are in the "controller" model
   Initial model "default" added

N.B. This deploys a 'Standard_D1_v2" instance type 

Trying the same on the default region (centralus) yields errors:

juju bootstrap azure cdk2

Creating Juju controller "cdk2" on azure/centralus
Looking for packaged Juju agent version 2.7.4 for amd64
Launching controller instance(s) on azure/centralus...
ERROR creating instance failed, destroying: creating deployment "machine-0": resources.DeploymentsClient#CreateOrUpdate: Failure sending request: StatusCode=400 -- Original Error: Code="InvalidTemplateDeployment" Message="The template deployment failed with error: 'The resource with id: '/subscriptions/bf4b9325-b972-4693-b28d-17b3226ab202/resourceGroups/juju-controller-model-4545acce-5105-4207-8566-07258b146404/providers/Microsoft.Compute/virtualMachines/machine-0' failed validation with message: 'The requested size for resource '/subscriptions/bf4b9325-b972-4693-b28d-17b3226ab202/resourceGroups/juju-controller-model-4545acce-5105-4207-8566-07258b146404/providers/Microsoft.Compute/virtualMachines/machine-0' is currently not available in location 'centralus' zones '' for subscription 'bf4b9325-b972-4693-b28d-17b3226ab202'. Please try another size or deploy to a different location or zones. See https://aka.ms/azureskunotavailable for details.'.'."
ERROR bootstrap failed, destroying model: cannot start bootstrap instance: creating virtual machine "machine-0": creating deployment "machine-0": resources.DeploymentsClient#CreateOrUpdate: Failure sending request: StatusCode=400 -- Original Error: Code="InvalidTemplateDeployment" Message="The template deployment failed with error: 'The resource with id: '/subscriptions/bf4b9325-b972-4693-b28d-17b3226ab202/resourceGroups/juju-controller-model-4545acce-5105-4207-8566-07258b146404/providers/Microsoft.Compute/virtualMachines/machine-0' failed validation with message: 'The requested size for resource '/subscriptions/bf4b9325-b972-4693-b28d-17b3226ab202/resourceGroups/juju-controller-model-4545acce-5105-4207-8566-07258b146404/providers/Microsoft.Compute/virtualMachines/machine-0' is currently not available in location 'centralus' zones '' for subscription 'bf4b9325-b972-4693-b28d-17b3226ab202'. Please try another size or deploy to a different location or zones. See https://aka.ms/azureskunotavailable for details.'.'."
^[[A^[[BERROR failed to bootstrap model: cannot start bootstrap instance: creating virtual machine "machine-0": creating deployment "machine-0": resources.DeploymentsClient#CreateOrUpdate: Failure sending request: StatusCode=400 -- Original Error: Code="InvalidTemplateDeployment" Message="The template deployment failed with error: 'The resource with id: '/subscriptions/bf4b9325-b972-4693-b28d-17b3226ab202/resourceGroups/juju-controller-model-4545acce-5105-4207-8566-07258b146404/providers/Microsoft.Compute/virtualMachines/machine-0' failed validation with message: 'The requested size for resource '/subscriptions/bf4b9325-b972-4693-b28d-17b3226ab202/resourceGroups/juju-controller-model-4545acce-5105-4207-8566-07258b146404/providers/Microsoft.Compute/virtualMachines/machine-0' is currently not available in location 'centralus' zones '' for subscription 'bf4b9325-b972-4693-b28d-17b3226ab202'. Please try another size or deploy to a different location or zones. See https://aka.ms/azureskunotavailable for details.'.'."

Bootstrapping whilst specifying an instance type works:

  juju bootstrap azure cdk2 --bootstrap-constraints="instance-type=Standard_D1_v2"


but there are still issues as the instance type problem reappears frequently when trying to deploy services/bundles.