How to fix mysqli_fetch_array() expects parameter 1 to be mysqli_result, boolean given in

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PHP

MySQL

I have this imported database in phpmyadmin with two tables and I am using codeanywhere. The tables are simple: First table i20_vrste(id, naziv), Second table i20_zivotinje(id, id_vrste, ime, starost, bolesna)

So, the connection file contains:

<?php
  $servername="localhost";
  $username="root";
  $password="";

  $conn=mysqli_connect($servername, $username, $password);
?>

and then my second file with included connection, sql query looks like this:

<?php
$sql="select * from i20_vrste,i20_zivotinje where i20_vrste.id=i20_zivotinje.id_vrste";
          $rez=mysqli_query($conn,$sql);
          while($niz=mysqli_fetch_array($rez)){
                $id=$niz['id'];
                $id_vrste=$niz['id_vrste'];
                $ime=$niz['ime'];
                $starost=$niz['starost'];
                $bolesna=$niz['bolesna'];
                $naziv=$niz['naziv'];
              if($bolesna==1){$bolesna="da";} else {$bolesna="ne";}
            
            echo "$id$naziv$ime$starost$bolesna";
?>

When I run it, I want to see the data from imported database but it pops this error mysqli_fetch_array() expects parameter 1 to be mysqli_result, boolean given in.

Why is this happening and what can I do to resolve this error ?

What I have tried:

I know that problem is in sql query probably, but please notice that I am a total beginner in php and I could use some help.