E: The repository 'cdrom://elementary OS 5.1 hera - stable amd64 (20200204) bionic Release' does not have a Release file.
In order to prevent this error you have to comment the line in your sources.list which asks for a CD Rom. An option to achieve this is by using command line.
1. Create a backup of the file first
Open your terminal application and paste or type the line
sudo cp /etc/apt/sources.list /etc/apt/sources.list.bak
The system will ask for your administrator password, you have to type it. If there is no error message, continue with the next step:
2. Comment out the line which asks for CD rom
Type or paste into termimnal:
and you will be into text edit mode of the system file I mentioned above. Using arrow keys of your keyboard you have to navigate to the line which starts with
deb cdrom:[elementary OS .. and put the
# sign in front of it in order to became
#deb cdrom:[elementary OS.. or
# deb cdrom:[elementary OS... Press ctl+o to save and then ctl+x to quit the application.