To benefit from Azure Defender you should use the Standard subscription tier.
Enabling Azure Defender extends the capabilities of the free mode to workloads running in private and other public clouds, providing unified security management and threat protection across your hybrid cloud workloads.
Using free subscription does not enable Azure Defender for the resource type
Enable standard subscription tier to benefit from Azure Defender
The following example will fail the azure-security-center-enable-standard-subscription check.
The following example will pass the azure-security-center-enable-standard-subscription check.