How to Install Nessus on Ubuntu 20.04
Installation Instructions for Ubuntu 20.04
Head on over to Download Nessus | Tenable® Click to Download Nessus-8.12.1-ubuntu1110_amd64.deb 
Open a terminal and navigate to where the .deb file was downloaded to. Run the command:
sudo dpkg -i Nessus-8.12.1-ubuntu1110_amd64.deb
If you have ufw (uncomplicated firewall) running enter:
sudo ufw allow 8834
Next we will enable and start the nessus daemon
#Use these two commands to enable then start Nessus
sudo systemctl enable nessusd
sudo systemctl start nessusd
In your web browser navigate to https://vulnscan:8834/
Choose Advanced -> Accept the Risk and Continue
Choose Nessus Essentials
Enter your First / Last Name, and email to receive and activation code
Copy and Paste the Code from your email into the Activation code spot and choose continue
Next Create a user account and then continue
Nessus will begin to Download and install necessary files
For extra information check: