sms/text 2010 iMac iMessage issue

I can use iMessage to message to other apple users but I cannot send text messages to others at all. My macbook air does them fine. Is this computer too old to do this? (like it wouldn't work... hahaha, thats a bad apple).

Can I or can I not sms with a 2010 iMac?

El capitan os.

thanks

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