Update dynatrace-free-trial.md

Added the link to purchase validation checks: https://learn.microsoft.com/en-us/marketplace/purchase-validation-checks

Author: vpriyanshi

articles/partner-solutions/dynatrace/dynatrace-free-trial.md

@@ -12,6 +12,8 @@ ms.subservice: dynatrace
 
 A 30-day free trial of Azure Native Dynatrace Service is available on [Azure Marketplace](https://azuremarketplace.microsoft.com/marketplace/apps/dynatrace.dynatrace_portal_integration?tab=Overview). You can sign up using the trial plan published by Dynatrace. During the trial period, you can create a Dynatrace resource on Azure and use integrated services such as log forwarding, metrics integration, and agent based monitoring. Before the free trial expires, you can seamlessly upgrade to a paid public plan or a private offer customized for your organization.
 
+Before you proceed, please make sure that your subscription is marketplace purchase enabled. For more information, visit [Purchase validation checks](https://learn.microsoft.com/en-us/marketplace/purchase-validation-checks)).
+
 ## Subscribe to a free trial  
 
 You can access the trial plan by finding Azure Native Dynatrace Service on Azure portal or in the Azure Marketplace. Refer to the guide to [create a new resource](dynatrace-create.md#find-offer) and choose the free trial public plan while subscribing.