Cannot create and access my Yahoo mail account on my old Catalina Mac mini.

Ian R. Brown wrote:

I have a Yahoo mail account on my 2017 Ventura iMac but it is not on my 2012 Catalina mini so I tried to add it.

I put in my email address and password and Yahoo sent the usual 6 digit verification code to my phone.

This was used but after a few moments the message "Unable to verify your account. Try again later" appeared.

I have tried again several times only to get a message about either the address or password being incorrect.

In spite of this my iMac is getting emails from Yahoo saying that there is a new device sign in and if it was me, I am good to go.

I tried this process a week ago with the same results.

What is wrong with Yahoo?

That is a question you'll need to ask Yahoo support or look at their help site.