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If you are facing this error try to run this command "# sudo nano /etc/apt/sources.list" I think this will give access to sudo and your error will be dimissed.
You can try this command to see if it fixes the problem. In my opinion, this will grant sudo access and make your mistake invisible.
It means that the source file needs sudo access. So try with sudo at front as shown below.
# sudo nano /etc/apt/sources.list
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