Cause

Why I can't modify resolve.conf is that it is linked as a symblic link.

Solution

All you have to do is in the following link that is written a solution you want to solve.

/etc/resolv.conf" E166: Can't open linked file for writing

Unable to update resolv.conf file. /etc/resolv.conf" E166: Can't open linked file for writing Ubuntu 18.04.1

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