Messaging to an android device
Why is it so hard to use iMessage to an android device
Why is it so hard to use iMessage to an android device
That's because the Mac and the iPad are not cellular phones... They can't send SMS or MMS messages by themselves.
You have to set up the connection between your Mac and/or iPad and your iPhone to send SMS and MMS messages through your phone.
Use Continuity to connect your Mac, iPhone, iPad, iPod touch, and Apple Watch - Apple Support
If I use my Mac or iPad to send to an Android through iMessage, it attempts but fails to send. If I use my iPhone it works fine. For all other messaging to other Apple users, I can use either Mac, iPad or iPhone to send and receive and it works fine.
As said above, sending SMS messages from a Mac or an iPad requires Continuity be setup so the SMS messages can be sent through the connected iPhone.
SMS messaging requires a cellular connection as it is entirely a cellular carrier service.
It isn't.
What issue are you having?
I can message android devices just fine. If the Android user also has an Apple device it may default to iMessaging that device instead however.
Thanks very much.
Messaging to an android device