This question relates to Ubuntu 18.04, Bionic The OP added, by mistake, an obsolete repository for Ubuntu 12.04, Precise, which has reached the end of its life. This obsolete repository does not exist any more and results in the error:
E: The repository 'http//archive.canonical.com precise Release' is not signed.
The solution is to remove the offending line from sources.list.
GUI method:
Open the program "Software Source"
Then go to the Other Software or Additional Repositories tab:
uncheck or remove the problems repositories
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Linux