I'm having the same issue in that Yahoo is saying it does not recognize my email. I have had that same email for almost 20 years now and now, out of the blue, I'm being told it isn't recognized.
OK, so it isn't recognized. By that, and per other people's comments, it means that my email does not exist. If that were the case, I should be able to create a new email. However, when I do so, it tells me that that email address is already in use.
I've emailed Yahoo about this and have yet to get a response. I need access to my emails. I have email notifications that are sent to me that I need to be able to see them in order to reply accordingly to them.
System Configuration: Windows / Chrome 75.0.3770.100