Hi @Dilan Nanayakkara ,
Thank you for the post! Make sure that you have the Account Owner role on the Enterprise Agreement enrollment and that you have a credit card on file. Those are the two things I can think of offhand that could impact your ability to add a new subscription.
You can follow the "Create an additional subscription" guide here: https://learn.microsoft.com/en-us/azure/cost-management-billing/manage/create-subscription
It is also free to speak with the Billing support team or create a support request with them if you are still having issues. https://azure.microsoft.com/en-us/support/options/