For this enrollment, you need an Azure AD Premium subscription. To verify if you have a Premium subscription, go to MDM enrollment configuration at https://portal.azure.com/#home. See the following example:
Azure Active Directory > Mobility (MDM and MAM) > Microsoft Intune
If this configuration doesn't appear, it's likely that you don't have a Premium subscription. Auto-enrollment is a feature only available in Azure AD Premium.
To convert your Intune trial account to a free Premium trial account, go to Azure Active Directory > Licenses > All products > Try / Buy and select Free trial for Azure AD Premium, or EMS E5.