Occasionally, when I attempt to send a picture/photo via Messages on my iMac, I get an error message of “message failed to send” and I have no idea why. In this instance, the photo is very small (about 430 kb), and is being sent to another iPhone user. In fact, it’s failed for sending to 2 different iPhone users. I’ve rebooted my iMac and my iPhone, but that hasn’t resolved the issue. I can send text messages with no issues.
MacOS 10.15.7, Message v13.0, iOS 14.4 - connected via ethernet (and WiFi) to the internet via fiber.
I’ve seen this for quite a while on my Macs. Considering how long it has been going on, I doubt it is a network issue. I can try the resend but it usually fails too. I think it is strictly bugginess that Apple has not fixed.
This has been my experience as well. I am currently on Catalina but it was happening before then. I had even done a nuke and pave at one point and this behavior continued.