Unable to connect to Google Cloud SQL PostgreSQL instance (private IP) using Go SQL Connector: iam.serviceAccounts.getAccessToken permission denied

I am trying to connect to a PostgreSQL database instance in Google Cloud (with a private IP connection) using the SQL Connector library in Go. However, I am encountering the following error:

ERROR Failed to dial instance {"error": "failed to get instance: Refresh error: failed to get instance metadata (connection name = \"<connection-string>\"): Get \"https://sqladmin.googleapis.com/sql/v1beta4/projects/\/instances/postgres/connectSettings?alt=json&prettyPrint=false\": compute: Received 403 `Unable to generate access token; IAM returned 403 Forbidden: Permission 'iam.serviceAccounts.getAccessToken' denied on resource (or it may not exist).
This error could be caused by a missing IAM policy binding on the target IAM service account.
For more information, refer to the Workload Identity documentation:

Environment Details:

Google Cloud: PostgreSQL instance with private IP connection enabled

Service Account Permissions:

• Cloud SQL Admin
• Cloud SQL Client
• Compute Network Admin
• Project IAM Admin
• Service Account Admin
• Service Account Token Creator
• Storage Admin
• Workload Identity User

Go Environment: Running the code in a VM instance that is part of the GKE cluster in the same VPC.

Here’s the relevant portion of my Go code:

func connectDB() error {
    user := "postgres"
    password := "password"
    connectionName := "connectionName"
    sslmode := "require"
    dbName := "databaseName"

    config, err := pgx.ParseConfig(connStr)
    if err != nil {
        return err
    }

    opts := append([]cloudsqlconn.Option{},
        cloudsqlconn.WithDefaultDialOptions(cloudsqlconn.WithPrivateIP()),
    )

    d, err := cloudsqlconn.NewDialer(context.Background(), opts...)
    if err != nil {
        return err
    }

    config.DialFunc = func(ctx context.Context, network, instance string) (net.Conn, error) {
        log.FromContext(ctx).Info(fmt.Sprintf("Dialing instance: %s", instance))
        conn, err := d.Dial(ctx, connectionName)
        if err != nil {
            log.FromContext(ctx).Error(err, "Failed to dial instance")
        }
        return conn, err
    }

    dbURI := stdlib.RegisterConnConfig(config)
    dbPool, err := sql.Open("pgx", dbURI)
    if err != nil {
        return err
    }   

    err = dbPool.Ping()
    if err != nil {
        return err
    }   
    return nil
}

Troubleshooting Attempts:

Verified that the database instance has a private IP and is reachable within the same VPC. Checked and ensured that the service account has the following permissions: Cloud SQL Admin Cloud SQL Client Service Account Token Creator Workload Identity User, among others. Ensured that the Go application is running inside a VM that has access to the private VPC.