It may be that when adding a new contact and trying to send a template, the following message appears:
"This number is not linked to a WhatsApp account and cannot receive messages."
This happens because the added number does not have an active WhatsApp account, therefore it will not be possible to interact by this means.
If you created the contact manually, make sure you check the number you entered and verify that it's correct.
If the client came through a campaign or online form, it's likely they left a landline (with no WhatsApp) or entered the wrong number.
When this happens, you can always try checking that the client's number is associated with a WhatsApp account so they can be contacted. To do this, go to wa.me/(client number) and WhatsApp will redirect you to your personal WhatsApp to send a message to the number you entered.
For example, if the contact number is +54 987-654 231, go to wa.me/54987654231, removing all hyphens and spaces.
You can also use this technique to create a contact link to your WhatsApp number that you can share with your clients, as explained in this article: "How to create a link to your WhatsApp number."
When you go to the address explained above with the client's number, most of the time you'll see the following message:
Note: Due to a restriction by WhatsApp/Facebook, some numbers when sending messages to or from the following countries (Crimea, Cuba, Iran, North Korea and Syria) will not enter Zenvia Conversion.