Can’t find the associated ABC external user

Is this a managed or supervised iPhone (associated with some organization)?

What is the exact text of the error message? (Post a screen shot, if no sensitive data is shown.)

Where are you downloading the app from? From the Apple App Store?

What steps and what troubleshooting have you already tried?

Steps to follow, followed by contacting Apple:

… If you can’t download or update apps on your iPhone or iPad - Apple Support

If you have an add-on VPN app, remove it, restart, and try again.