I tried this:

$ az deployment group create --resource-group glock-rg \
                             --template-file template.bicep \
                             --parameters @parameters.json

The selected VM size 'Standard_A1_v2' cannot boot Hypervisor Generation '2'. If
this was a Create operation please check that the Hypervisor Generation of the
Image matches the Hypervisor Generation of the selected VM Size. If this was
an Update operation please select a Hypervisor Generation '2' VM Size.

It turns out this is because my properties.storageProfile.imageReference.sku was 18_04-lts-gen2 - the gen2 was the incompatible part. To find all the compatible VMs:

$ az vm image list-skus --location westcentralus --publisher Canonical --offer UbuntuServer --output table
Location       Name
-------------  --------------------
westcentralus  12.04.5-LTS
westcentralus  14.04.0-LTS
westcentralus  14.04.1-LTS
westcentralus  14.04.2-LTS
...

See this page for the full documentation.