The azurepipeline does not currently have support for the ArmPlatformPkg. When submitting a pull-request, no tests are run. Having the ArmPlatformPkg part of the CI would be an enhancement. Cf: https://github.com/tianocore/edk2/blob/master/.azurepipelines/templates/pr-gate-build-job.yml
Pierre is working on it.
Resolved by: https://github.com/tianocore/edk2/commit/a4cf19597f179a6af7dc56cf0c20e2f2f834db77 and https://github.com/tianocore/edk2/commit/1cb1087749ad3281bc65311849028db70d4b9739