Can’t sign into Microsoft 365 to set up new laptop

My son has a new laptop and is trying to set it up. He is a family member on our Family Microsoft 365 subscription. He has logged out of the old device and removed the device from the Microsoft 365. When he tries to set up the new ASUS laptop, an error message appears after he tries to authenticate on his phone and he can’t get any further in the set up.

we are in a loop for an hour! How do we resolve this? We can’t get into any screen on laptop other that the Microsoft 365 sign in screen.