If you set your clients to be paying offline, it simply means they are paying you via other means and not through Kahunas's payment gateways.
If you would like to start receiving payments through Kahunas, you would first need to link your account to PayPal or Stripe.
Once done, Go to clients > all clients > select the desired client > edit, scroll down, and click on next, Assign them a payment plan/package, choose "yes" under “Does this client need to pay?”
This client won't have access to the app unless they pay by logging in to Kahunas's website, they will be prompted to make a payment.
Alternatively, you can click "actions" in front of the client's name and click on "send payment link", which will send the client a payment link via email.