Ah, the problem with "I've tried everything" is that it doesn't tell us what you've tried. You've uninstalled and reinstalled the app? Force stopped then clears the app's cache and data? Checked sync settings? Checked that doze isn't interfering with its sync?
I don't have a Yahoo account so don't know about the Yahoo app, but it's also possible that the app simply needs updating for Android 7 (incompatibilities with new OS versions are not unknown, and I'm still getting app updates where the description says "fixes compatibility with Android 7" or similar). We'd need someone who uses this app with this OS version to confirm whether it's working for them or not.
