Similar to this question about Ubuntu 20.04, the list of additional drivers is greyed out and I can not change the driver. Currently "continue using a manually installed driver" is selected.
I first installed nvidia-driver-535, I also managed to install the cuda drivers for it.Then I wanted to change the driver to nvidia-driver-550. So I selected the driver and hit "Apply Changes". During the installation, an error message appeared saying that something had failed. I didn't understand what the problem was and I also forgot to take a screenshot. The problem with that error message was that I could not close it but using this question I was able to somehow reach the "close" button.
But then the problem of this question happend.