https://www.tensorflow.org/install/pip#linux

Install pip:

$ sudo apt update
$ sudo apt install python3-pip

Then follow the guide to install miniconda and virtual environment tf.

==== install Nvidia driver

Before installing CUDA and etc. for GPU, check if nvidia GPU driver is installed:

$ nvidia-smi

If no result, find compatible drivers list:

-------- not working begins
$ ubuntu-drivers devices

Select 535-server. (the latest one)

$ sudo apt install nvidia-utils-535-server

But nvidia-smi does not work.
Remove 535
$ sudo apt remove nvidia-utils-535-server
$ sudo apt autoremove
-------- not working ends

Download NVIDIA-Linux-x86_64-535.98.run from the link in tensorflow installation page.

During runnning the .run, Nouveau kernel driver is disabled with the following message:
One or more modprobe configuration files to disable Nouveau have been written. For some distributions, this may be sufficient to
disable Nouveau; other distributions may require modification of the initial ramdisk. Please reboot your system and attempt NVIDIA
driver installation again. Note if you later wish to re-enable Nouveau, you will need to delete these files:
/usr/lib/modprobe.d/nvidia-installer-disable-nouveau.conf, /etc/modprobe.d/nvidia-installer-disable-nouveau.conf

But after reboot, the .run fails again.
Update ramdisk file system(??) with:

$ sudo update-initramfs -u

And then reboot again. Now .run installation works.

==== And then install CUDA and etc. by the guide of tensorflow.org