The account recovery process is completely automated. Yahoo determines you are the rightful owner of the account by sending the account key code message to the verified phone number or email address entered on the Account Security page for this account. If this information is outdated, was entered incorrectly, or was never set up then it's unlikely you will be able to recover this account.
Start by going through the Sign-in helper mentioned in the following article from a laptop or a desktop.
Problems signing in to your Yahoo account -
When you go through the account recovery process and you see account recovery options that are no longer available you need to choose "I don't have access to this phone" or "I don't have access to this email" to see what other account recovery options are available. If you get the "Uh oh" message saying the account can't be recovered online you can try the following but it's unlikely you will be able to recover this account.
Start here at Help for Yahoo Account - https://help.yahoo.com/kb/account
Scroll down the page until you see a "Contact Us" button.
Click on this to open a page that says "Need assistance with a Yahoo product? Let's see how we can help.".
Under "Tell us your Yahoo email address and product." you select the product you want help with. In this case it would be "Account".
Enter your email address and check "I'm not a robot". You may be sent to another CAPTCHA page for additional proof.
Click the "Get started" button
This opens the "CHOOSE SUPPORT OPTION" page.
Under "Select from these options" you will find all the support options available to address this issue.