How to _GET multiple value checkbox WP_Query in Custom Toxonomy / Custom Fields

your checkboxes have the same id, the id is unique, otherwise would be a class.
in the forms the checkboxes should use each one an unique id and name,check this example at w3schools.com
then when checking for the $_GET we should use the isset because if for some reason does not have that $_GET will return an error in the code.

if(isset($_GET['minprice']) && !empty($_GET['minprice']))
  {
     $minprice = $_GET['minprice'];
  } else {
     $minprice = 0;
  } 

and the !empty($_GET[‘minprice’]) should be set in the html instead as requiered like so

<label>min:</label>
<input type="number" name="minprice" value="<?php echo $minprice; ?>" required >

all this should putt your code working, I even had check nothing more because this should respond to your question, if some issue more appear give feedback.