New MacBook Air M1 says I have to "enable iMessages" to send texts to non-Apple devices. I can send and receive iMessages but not texts. How do I "enable" iMessages on my new MacBook Air?

Thanks. I had gone through these steps earlier today and went back through them again now, and kept my iPhone next to the MacBook and it seems to have worked. I had sent pictures before as a text to one person and as a Message to another and it worked, and it worked again now, so not sure why it didn't earlier today. Maybe the devices being in different areas of someone's house and a less than strong wi-fi signal was an issue? I don't know. The only other difference I could tell was that the earlier attachments were ones I had copied from another message and these were from my photos on the MacBook. I'm a long-time (45 year) PC user, not having an Apple since my Apple IIe, so I'm still learning on the MacBook Air.