NVIDIA driver is not loaded error

I have Ubuntu 18.04 and use following command to install Nvidia driver
sudo apt-get purge nvidia*
sudo add-apt-repository ppa:graphics-drivers
sudo apt-get update
sudo apt-get install nvidia-driver-440

after rebooting, I run lsmod | grep nvidia and get following
i2c_nvidia_gpu 16384 0

when I run prime-select query, it outputs nvidia

when I run nvidia-settings, it outputs Error: Nvidia driver is not loaded

I run mokutil --sb-state and the output is SecureBoot disabled

lastly , I run nvidia-bug-report.sh to generate report, which is attached. anyone can suggest why it says NVIDIA driver is not loaded?nvidia-bug-report.log.gz (119.7 KB)