How to sovle if predefined function is not accepting

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PHP

I'm using vscode and xampp for my project
but predefined functions are not been accepted how to resolve it as soon as possible
can anyone help me as soon as possible
i have to submit my project on monday

What I have tried:

$query = "INSERT INTO user VALUES ('$rn','$re','$rc')";
$data= mysqli_query($conn,$query);