By default ConvertKit asks subscribers to confirm their subscription via an email message (you'll often hear this referred to as double opt-in). This ensures that someone really wants to be on your list and generally results in a cleaner list.
If you want single opt-in, so that anyone who enters an email address is automatically confirmed without further action, go to the form or landing page you're using, click the Email message icon to get to the Incentive Email Message settings, and find the checkbox:
Auto-confirm new subscribers
Once you've checked this box, new folks will be automatically confirmed.
You cannot email unconfirmed subscribers.