Please check the below steps and do let us know.
- Eligibility: The Microsoft Purview solutions trial is available to Microsoft 365 E3, Office 365 E3, and Enterprise Mobility and Security E3 customers who don't already have a Microsoft E5 license package If your organization already has an E5 license, you might not be eligible for the trial.
- Government Customers: The trial isn't available for Microsoft 365 Government customers If your organization falls under this category, that could be the reason.
- Role Requirements: While global admin rights should generally allow you to start the trial, there might be specific role requirements or permissions needed Ensure that your role includes the necessary permissions for starting the trial.
- Technical Issues: Sometimes, technical glitches or delays in the system might prevent the trial from being activated immediately. It might help to sign out and sign back in, or wait a couple of hours and try again
I hope the above steps will resolve the issue, please do let us know if issue persists. Thank you