The Messages app on my Macbook Pro (running High Sierra) suddenly stopped sending text messages to Android devices. I can still send texts to Android users from my iPhone without a problem, but when sending texts from the Messages app on my Macbook Pro it will not send to Android devices. It just started doing this out of the blue yesterday. I have not had an issue until then. And I have not made any updates to the Mac or the iPhone OSs in that time that might cause the problem, but I did install Microsoft Office 2018 on the Mac. Cellular data is turned on on my iPhone as is Send as SMS.
Text messages from your Mac are actually sent through your iPhone. On your iPhone go to the Settings app → Messages → Text Message Forwarding. You may need to re-do the setup by removing and re-adding your Mac in there.