Messages on macOS not working with SMS messages
When sending SMS messages from my Mac, I'm getting "failed to send" messages. When I send them from my iPhone, they don't show in Messages. I'm using iCloud Message and it was working until today.
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Try rebooting the computer.
If that does not work, then try toggling Messages off, then back on from the iPhone.
If that still does not work, then make sure message forwarding has been enabled on the iPhone.
Forward SMS/MMS text messages from your iPhone to other devices - Apple Support
Try rebooting the iPhone by completely powering down your iPhone, then power it back on. I would highly recommend closing the Messages app on the iPhone prior to rebooting the iPhone.
Are both your iPhone and macOS on your computer completely up to date with all the update patches installed?