MMS Messaging from mi iMac

I assume you are trying to do MMS messaging from the Mac. That would be needed, for instance, to send a Message (text message) from your Mac to a non-Apple device.

To do this you have to have the iPhone iMessage setup linked to the Mac Messages setup. The option for "Text Message Forwarding" on the iPhone enables what you are trying to do from your Mac. Without this setup as described below, the Mac can only send Messages to other Apple devices through your Apple ID, but no MMS through the Mac without the iPhone setup as indicated below.

Here is a guide to setting this up. Each step must be followed, without exception. (If you don't have an iPhone, the Mac can only send Messages to other Apple devices through your Apple ID, but no MMS messaging.)

iMessage Guide and Checklist:

iPhone and Mac both logged in to same Apple ID. Try logging off and logging on if necessary.

iPhone Settings => Messages shows iMessage as "on" (sometimes toggling this off and then on fixes some things)

iPhone Settings => Messages shows Send & Receive showing both phone number and email (Apple ID)

iPhone Settings => Messages shows Text Message Forwarding and your Mac is listed and shown as "on" there.

iPhone Settings => Messages shows Send as SMS "on" and MMS Messaging and Group Messaging both "on"

On the Mac: Messages => Preferences => iMessage tab shows same and correct Apple ID and "You can be reached for messages at" indicates both phone number and Apple ID (email), and "start new conversations from" indicates the phone number.

* Don't see the Text Message Forwarding setting on your iPhone? Go to Settings > Messages, turn off iMessage, turn it back on, tap Send & Receive, tap Use Your Apple ID for iMessage, then sign in with the same Apple ID used on your other devices.

Forward SMS/MMS text messages from your iPhone to your Mac or iPad - Apple Support