Hi,
If this is a new account under Microsoft Online Services Program you should be limited to a total of 5 subscriptions. Do you have 5 subscriptions (Pay-As-You-Go or other) associated with your account?
https://learn.microsoft.com/en-us/azure/cost-management-billing/manage/view-all-accounts
If you need to create additional subscriptions and are being blocked, I recommend you open a new Billing/Subscription Management Support Request using link below:
https://portal.azure.com/#view/Microsoft_Azure_CostManagement/Menu/~/support/openedBy/AzurePortal
It will look similar to below sample screenshot:
Enter summary, select Problem type, select Problem subtype, then click Next. After a delay, some potential solutions will be shown. Click Return to support request as shown below:
This will return you to the recommended solution tab. Click Next, as shown below:
Fill out the information on the Additional details tab making sure your email address/phone are correct, click Next, then finally click Create. At this point your support request will be created and you should receive a Case email from Microsoft Support and will be contacted in near future by support engineer.
If the above information is useful please Accept Answer.
Thanks.
-TP