Hi corvet11,
welcome to the Apple (user-to-user) Community.
Re: "Old iPad--software is 12.5.7. I deleted the Yahoo mail app
and am trying to reinstall this app"
Seems the latest Yahoo Mail app in the App Store may not be compatible with your older operating system, if that is the app you've been trying to install:
Apple's article for iPadOS 12:
How to: Get apps and games in the App Store on iPad – Apple Support
An alternative:
Wondering if you may be able to just add your Yahoo eMail account to the Mail app on your iPad instead, in order to restore access your Yahoo eMail.
Eg: For iOS 12: In Change the iPad settings – Apple Support
You can: scroll to: Set up mail, contacts, and calendar accounts
Eg: says (On iPad):
Go to Settings > Passwords & Accounts > Add Account.
To add a mail account, tap an email service—for example, Google, Yahoo, or Aol.com—then enter your email account information.
NB: If you are missing any information that needs to be entered:
You could likely log-in to your eMail account on the Yahoo website, and retrieve whatever you need. (Eg: If you forgot the password, you can usually get a new one from the log-in webpage.)
All the best :-)