Thanks for reaching out to us! We currently do not support the use of an alternative texting application, as the messages are not being handled in a traditional method they do need to be initiated and completed by the TextNow application directly.
The error you are receiving is being cause by an application trying to submit an SMS using traditional processes via the cell network. Service with TextNow does not work in this same method, and everything is powered by technology and leverages data connections.
Reviewing your account, it appears that you are using a TextNow device, so please contact us directly so that we can review all of the settings in the device and ensure that everything is working properly.
I hope that answered all of your concerns/questions!