If the app is installed but shows an "Error loading, tap to retry" message, it is because YouTube's servers no longer recognize the old version. You can fix this by spoofing the version number.

Plug your iPhone/iPad into your computer. Unlock the device and tap “Trust This Computer.”

(in iOS 10.3.4 YouTube IPA)

Apple’s iOS 10.3.4 holds a bittersweet legacy. Released in July 2019, it was a critical GPS fix for older devices, but more importantly, it became the for some of Apple’s most beloved hardware—the iPhone 5 and iPhone 4s. However, there is a dark cloud looming over these vintage devices: App Incompatibility.

For the uninitiated, an (iOS App Store Package) is the archive file for an iOS application. Think of it as a .exe for Windows or a .dmg for Mac.