While creating your SDDC and connecting a VPC and subnet to connect to in your AWS account, you are unable to select a subnet.
While deploying an SDDC, there is a step in which you select an Amazon VPC and subnet in your AWS account to connect to your SDDC. You might be unable to select a subnet during this step. A message in the UI indicates that you do not have capacity in any of your current subnet AZs.
You must select a subnet in the same availability zone (AZ) as your SDDC. Currently, it isn't possible to ensure which AZ your SDDC will match up to. If you have only created a single subnet, it might be in the incorrect AZ and not available for selection in this step.