Keep it simple. Please start with only the AMD GPU.
If you have a laptop, just disconnect the external monitor.
If you have a desktop, just move the monitor connection to the motherboard. You might have to also temporarily remove the Nvidia card during that time or else the system will still prefer Nvidia even while no monitor is connected to it.
Once you can use the machine with only AMD graphics, you can install the Nvidia driver manually (even when no Nvidia card is installed):
sudo apt install nvidia-driver-510
and reboot. Then it should be safe to go back to using Nvidia and multi-monitors.
If you're installing Ubuntu from scratch then the shortcut to all this is to choose the third-party software option during installation.
I am of course guessing that the problem is the nouveau driver and not AMD...
Keep it simple. Please start with only the AMD GPU.
If you have a laptop, just disconnect the external monitor.
If you have a desktop, just move the monitor connection to the motherboard. You might have to also temporarily remove the Nvidia card during that time or else the system will still prefer Nvidia even while no monitor is connected to it.
Once you can use the machine with only AMD graphics, you can install the Nvidia driver manually (even when no Nvidia card is installed):
sudo apt install nvidia-driver-510
and reboot. Then it should be safe to go back to using Nvidia and multi-monitors.
If you're installing Ubuntu from scratch then the shortcut to all this is to choose the third-party software option during installation.
I am of course guessing that the problem is the nouveau driver and not AMD...