Bootstrap the CDK into your account and region. This is a one-time operation that creates the resources necessary to allow CDK deployments in your AWS account to a given region.
You should see messages from CDK indicating that it bootstrapped your account and region:
Bootstrapping environment aws://111111111111/us-west-2... CDKToolkit: creating CloudFormation changeset... ... Environment aws://111111111111/us-west-2 bootstrapped.