Oh, OK, from your response, I can surmise the following:
- You own an iPhone.
- You are attempting to send the message(s) to a telephone number.
So based on that, I will assume the telephone number is not associated with an iPhone (because if it were, the addressee would be blue indicating an iMessage).
So presumably you're attempting to send an SMS text message, which a Mac cannot do, but it can hand off to an iPhone to perform. Be certain, therefore, you have set this up correctly.
How to forward SMS/MMS text messages from your iPhone to your iPad, iPod touch or Mac - Apple Support