A number from my existing address shows red in to line to send. It will not send. Why?
Why does red text show in "to" line and will not send on iPad?
iPad Pro Wi-Fi
The most likely reason is that the iPad is not a cellular telephone, and therefore is incapable of sending SMS text messages to non-Apple cellular telephones. The iPad can only send iMessages to other Apple devices.
There are other possible reasons, such as an incorrectly formatted telephone number for your country or region, but the reason above is certainly the most likely.