For example, I use the / testfor @s and coordinates command to not search for a player. This error is also in other commands that support @s. I have it on the day that it will be repaired because I can not end up with two maps.
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@s means "the thing running the command."
So by doing /testfor @s, you are testing for the command block. Command blocks aren't entities so it intentionally fails.
Are you running the command in a command block?