How can I activate IPv6 for a currently running EC2 instance and establish an SSH connection utilizing IPv6?

Hello.

First, you need to set IPv6 CIDR in your VPC by following the steps in the document below.
https://docs.aws.amazon.com/vpc/latest/userguide/vpc-migrate-ipv6.html

Once you have set up IPv6 CIDR, please configure your route table and security group to support IPv6.
https://docs.aws.amazon.com/vpc/latest/userguide/vpc-migrate-ipv6.html#vpc-migrate-ipv6-routes
https://docs.aws.amazon.com/vpc/latest/userguide/vpc-migrate-ipv6.html#vpc-migrate-ipv6-sg-rules

Once the settings are complete, configure IPv6 on the EC2 network interface using the steps in the document below.
Depending on the OS, you may need to configure settings to enable IPv6 on the interface.
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/using-instance-addressing.html#working-with-ipv6-addresses

@Riku_Kobayashi Please review my security group and Internet Gateway settings as I am encountering difficulties connecting to my EC2 instance using SSH over IPv6.