unable to create t3 burstable instances with custom VPC

I created a custom VPC with 9 subnets, such that for 3 AZ I have 3 subnets (public, private, and persistence). It has the following configuration:

VPC

10.100.0.0/16

Subnets

- 10.100.0.0/24 (az1) - 10.100.10.0/24 (az1) - 10.100.20.0/24 (az1) - 10.100.1.0/24 (az2) - 10.100.11.0/24 (az2) - 10.100.21.0/24 (az2) - 10.100.2.0/24 (az3) - 10.100.12.0/24 (az3) - 10.100.22.0/24 (az3) 

Everything seems okay, except I am unable to create any type of burstable instance within my VPC.

In the AWS web UI, if I choose a t3.X instance, then I am only able to choose the default VPC and my custom VPC is greyed out.

Likewise, I am unable to create an RDS burstable instance. I created a subnet group which consists of the persistence subnets (e.g. 1 private subnet from each AZ defined above). However, when I attempt to create the DB, I get the following error:

Cannot create a db.t3.medium database instance because there are no availability zones with sufficient capacity for VPC and storage type : gp2 for db.t3.medium. Please try the request again at a later time.

I am able to create all other types of instances in EC2 and RDS (e.g. m, r, and x classes).

Does anyone know what's going on?

1. I have no other instances launched in the VPC, so I know that I have plenty of addresses.

2. I am able to launch the tX instance when I use the default VPC provided by AWS. It's only when I try to launch in my own instance that it fails. I assume this means the devices exist in the region/AZ, so I don't think it's a region capacity issue.