Please note to check the type of your subscription and make sure it's not a free subscription.
Q: What types of Azure subscriptions can I use to buy Azure DevOps?
A: Almost all Azure subscriptions. We support Azure subscriptions connected to your Enterprise Agreement (EA), Azure subscriptions
set up by Cloud Solution Providers (CSPs), through Microsoft Open
License resellers, and Pay-As-You-Go. You can also buy using Azure
subscriptions that Visual Studio subscribers set up as a subscriber
benefit. (But no, your monthly credit can't be used pay for more
Visual Studio subscriptions.) The only notable exclusion is that you
can't use the Azure free trial, Government, or National clouds.
Besides, if you have used a paid sub, please report your issue to Developer Community.