Can't send text messages from my macbook air.

Greetings rfedell, 

You’ve mentioned this is for text messages. Does that extend to both iMessages and SMS messages? 

It also looks like you came to Communities through this article If you can’t send or receive messages on your Mac which has great steps including checking for Text Message Forwarding being enabled, checking that your iMessage activation is complete, etc. Did you have a chance to work through all those steps? 

Also helpful to note is what happens when you attempt to send a text message from your Mac? If there are any error messages, what do they say? 

If this issue impacts only SMS messages, messages to non Apple devices, let's check your Settings. If any settings are incorrect, this may be why you can't receive text messages: How to forward SMS/MMS text messages from your iPhone to your iPad, iPod touch or Mac - Apple Support

Set up text message forwarding

1. On your iPhone, iPad, or iPod touch, go to Settings > Messages > Send & Receive. On your Mac, open Messages, choose Messages > Preferences, then click iMessage. Make sure that you're signed in to iMessage with the same Apple ID on all your devices.
2. On your iPhone, go to Settings > Messages > Text Message Forwarding.*
3. Choose which devices can send and receive text messages from your iPhone. If you're not using two-factor authentication for your Apple ID, a verification code appears on each of your other devices: enter that code on your iPhone.

As long as your iPhone is turned on and connected to Wi-Fi or a cellular network, new SMS/MMS texts can be sent and received on the devices that you added.

iCloud now keeps your entire messages history updated and available on all your devices — even when you set up a new device. Learn how to keep all your messages in iCloud.

* Don't see the Text Message Forwarding setting on your iPhone? Go to Settings > Messages, turn off iMessage, turn it back on, tap Send & Receive, tap Use Your Apple ID for iMessage, then sign in with the same Apple ID used on your other devices."

As an extra step, toggle off your Mac under Text Message Forwarding. Then turn it on. You should be prompted to confirm things. At that point quit and relaunch the Messages app on your Mac and try again. 

Thanks.