The CBA package for basic_vm_macro uses variable names with lower case names,
wheres in the used HEAT-file uppercase variable names are used.
Therefor you cannot override the variables and the testcase (e.g. in DT-lab fail).

CBA package "templates/artifacts/basic_vm_cba.zip" (e.g. base_ubuntu20-template.vtl):

    {
      "param-name": "ubuntu20_image_name",
      "param-value": "${ubuntu20_image_name}"
    },
    {
      "param-name": "ubuntu20_flavor_name",
      "param-value": "${ubuntu20_flavor_name}"
    },

In the HEAT-file "templates/heat-files/ubuntu20/ubuntu20agent.zip" (e.g. base_ubuntu20.env)

# Server parameters, naming required by ONAP
 Ubuntu20_image_name: ubuntu-20.04
 Ubuntu20_flavor_name: m1.small

Fix would be to update the HEAT-File to use only lower-case "Ubuntu*"