Network - Likely bad network (debug by checking if you are able to do other things and receive new emails etc)Try after sometime.Not sending mails could mean a lot of things, the UI is saying cannot send, it hangs, it sends but the recipient is not getting it .. in which case there might be an issue with the recipient.Contact yahoo support they will help you, I am sure you will not get a great support from Quora on this.
I was also able to receive snaps and chats but was not able to send messages to my friends and was getting the same error as failed to send - tap to try again. Thankfully, the following article helped me resolve this issue once and for all:How to fix Snapchat Failed Send ErrorHope it helps. Thanks.
you have to check what are you doing, maybe you are doing something wrong, go to this site How to Send a Message, Video, Image and File with Yahoo
The issue could be that the "account stopped syncing" due to some reason i.e. password change etc.Apparently, the previous answer was wrong because send button is enabled even with an empty To field.( Previous answer: Please add an email ID in the To field and the send button should magically be activated!Edit-1: There is no point of send button being enabled if there is no recipient added in To field. It is safe to keep it disabled to avoid unexpected scenarios.)