The cause might be one of following issues.
1. The menu is not provided in that region or group.
Please refer to below for the list of services provided in each region or group. Service Descriptions - The List of Services provided in each Region
2. Creating a volume is not available temporarily.
(1) Please try it again later.
(2) Please consider creating a volume in another region / another group.
(3) When the issue is not resolved by the above actions, due to popular demand, we are afraid that it might not be possible to do resource creation in the region/group where the customer is trying to do it. In that case, please create a ticket via Ticket System.