Hello,
I hope you followed the steps shared in the document when trying to upgrade your subscription using the same account that was used to create the free trial account. Please try once again following the steps to upgrade.
If you still face issue, follow the steps shared in below document section to create Azure Subscription and billing request:
https://learn.microsoft.com/en-us/azure/azure-portal/supportability/how-to-create-azure-support-request#go-to-help--support-from-the-global-header
If you're unable to create support request, try to reach support via global customer support as shown below.
https://support.microsoft.com/en-us/topic/global-customer-service-phone-numbers-c0389ade-5640-e588-8b0e-28de8afeb3f2
Ensure to share your preferred contact method and all the details about your query/issue and the team will connect with you.
Hope this helps!
Regards
Divya.