Fatal error: Call to a member function prepare() on null
Solution 1:
It looks like your $pdo variable is not initialized.
I can't see in the code you've uploaded where you are initializing it.
Make sure you create a new PDO object in the global scope before calling the class methods. (You should declare it in the global scope because of how you implemented the methods inside the Category class).
$pdo = new PDO('mysql:host=localhost;dbname=test', $user, $pass);