Messages: "Your message could not be sent"
I am on a MacBook Air (see specs below), and I have Messages set up on my iPhone and computer. I am obviously on my phone more often than my computer, and every time I open my computer, it takes a couple of minutes for the MacBook to "catch up" with my phone in terms of messages sent and received.
For about 2 - 5 minutes after I open my computer, I get continuous notifications of messages I have already seen and responded to on my phone. Annoying as it is, I just accepted it as part of the territory. However, after installing and using some of the iMessage apps (stickers, GamePigeon, etc.) I started getting the message: "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]" (See image)
I have tried clicking all three options, and the pop up continues to show; I'm talking at least 100 times at this point. No matter which selection I make, within 10 seconds, it pops back up.
I have tried signing out of my iMessage account, closing Messages, waiting 5 minutes, and restarting Messages, but the problem persists. Somebody please help!
<Image Edited by Host to Remove Personal Information>
MacBook Pro, macOS Sierra (10.12.1)