I clicked the "Request Trial Capacity" button 20 minutes ago in a scratch org and Get Started with Platform Cache says:
Click Request Trial Capacity and wait for the email notifying you that your Platform Cache trial is active. Salesforce approves trial requests immediately, but it can take a few minutes for you to receive the email.
but no email and still a "Trial Request Pending" message in the refreshed page.
Is more patience needed, or are scratch orgs not supported?