How to delete this PPA repository?

unfortunately, you didn’t give all the information that apt-get gave you. there should have been indicated the name of the repository with which problems arose.

judging by the references to the line C2518248EEA14886 found on the Internet, this key signifies packages either in the webupd8 repository, or in the vlc repository.

To “turn off” the repository, you can, for example, comment out the lines in which these words occur ( webupd8 or vlc ). the necessary files can be found, for example, like this:

 $ grep -rlE '(webupd8|vlc)' /etc/apt/sources.list* 

“Comment out” is to insert the # character at the beginning of a line.

addition

if the search words were not found, you should look for the words ppa or trusty :

 $ grep -rlE '(ppa|trusty)' /etc/apt/sources.list* 

due to the fact that the debian distribution does not use the default “left” repositories (from the same, for example, the site ppa.launchpad.net ), and trusty is the codename of one of the releases of the other distribution, ubuntu , you hardly have many . try to comment them all out. or selectively, until the error disappears.