ERROR: Could not find a version that satisfies the requirement tensorflow-addons<0.8.0,>=0.7.1 (from rasa) (from versions: none)
I faced same problems and executed following commands:

sudo apt-get update
sudo apt-get upgrade
sudo pip3 install --upgrade pip
pip3 install rasa

Now My System is:

Ubuntu 18.04.5 LTS
Python 3.6.9
pip 20.3.3
tensorflow 2.3.1
tensorflow_addons 0.12.0

and rasa on my system is working fine.
I tried to install Rasa, with the command: pip3 install rasa.
However, I came up against an error about tensorflow, which is not automatically installed.
Then I used command: pip3 install tensorflow, unfortunately, an error appeared:
ERROR: Could not find a version that satisfies the requirement tensorflow-addons<0.8.0,>=0.7.1 (from rasa) (from versions: none).
So how could I install Rasa, and Tensorflow as well?
P/s: Python --version: 3.8.3;
pip --version: 20.1.1