I continuously have "Failed to send" messages in chat (individual or group), however, I might be in a Teams call or video and that keeps working with no issues. The only workaround I found is to switch WIFI (from 2.4GHz to 5.8GHz or viceversa), which disconnects me from the active call for 10 seconds. Then I'm able to press "Retry" in failed messages which are sent with no issues, until the problem occurs later again. This happens to me around 10 times per day.
I'm connected in a home internet, with a strong signal (up to 300 Mbps), and not using VPN. The error happens also within the work VPN. Not sure this is a Teams error, a router issue or a network configuration one.