Can't text

There are a couple of reasons that could have happened. Your friend could have signed out of iMessage. Or, your friend could be somewhere where there is no data (WiFi or Cellular). Do you have another way of reaching them to ask?

Also, do you have an iPhone? If so, if you connect your iPhone to your Mac, you will be able to send messages even to people who are not using iMessages. See here:

Forward SMS/MMS text messages from your iPhone to your Mac or iPad - Apple Support

"Can't text: So I have a MacBook Air and my friend has a iPhone 11 and we were texting and all of a sudden his contact turned red and nothing delivers anymore and I can't fix it. What do I do?"

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If Contact Goes Red:

Add/Remove: First, add the Contact, and then add the Contact back. Take a screenshot of the Contact's credentials, prior to removing the Contact (so that you can have something to refer to when adding the Contact back). Also, ask the Contact if they not longer have the service of Messaging.