If it just says “failed to send” it’s something wrong on the Microsoft side. There’s been problems with this during the whole last week due to problems with the service.
There is moderation for certain channels that can disallow members from posting or replying in a certain channel but this should go for all members , always and also say so, not give an error message.
Check the service health If you can access admin portal. Wait for time and it will probably resolve itself otherwise create a ticket with Microsoft
Adam