Error: Could not resolve 'bionic' unable to find package
These problems occur when sources repository address is configured wrong.
To fix this problem sudo nano /etc/apt/sources.list
find out which repository is causing problem and remove it.
To add add default repositories use below command:
sudo add-apt-repository main
sudo add-apt-repository universe
sudo add-apt-repository restricted
sudo add-apt-repository multiverse
And finally do an sudo apt-get update -y