iMessage: Your message could not be sent.

I just got back from vacation and my friends and I had been chatting for the past week using iOS Devices. When I turned on iMessage on my iMac, I see an error message saying "Your message could not be sent. There was an error sending the previous message. Would you like to try to send it again? [Ignore] [Go to Chat] [Resend Message]"

I figured it wasn't a big deal and just hit the ignore button. But then I saw that all images and videos that my friends and I had sent in the chat were "Not Delivered" and in a weird file form like it wasn't downloaded and the download button didn't do anything.

I don't understand how it says "Your message can not be sent" when it did send and was received on other devices. All the videos and photos are fine on my iPod. I saw some other posts on Apple Discussions with similar errors, but not relatable enough for me to use the fixes provided. I don't have any iMessage add-ons for the iMac. iPod and iMac are up-to-date. Here's some screenshots of iMessage on the Mac:

This problem is very annoying and persistent. Please help!