Hello, I installed it on my version of netbeans and phpunit phpunit 8 by composer-skeleton.
Then I created a sample class called teasing calculator.php 

//example
<?php

class Calculator
{
    /**
     * @assert (0, 0) == 0
     * @assert (0, 1) == 1
     * @assert (1, 0) == 1
     * @assert (1, 1) == 2
     * @assert (1, 2) == 4
     */
    public function add($a, $b)
    {
        return $a + $b;
    }
}
?>
 
and when I click with the right mouse button and click on "Tools -> Create Test" console gives me this and I do not generate the test:

"C:\wamp\www\annunci\vendor\bin\phpunit-skelgen.bat" "--test" "--" "Calculator" "C:\wamp\www\annunci\app\models\Calculator.php" "CalculatorTest" "C:\wamp\www\annunci\test\app\models\CalculatorTest.php"
phpunit-skelgen 2.0.1 by Sebastian Bergmann.
                                     
  [InvalidArgumentException]            
  Command "Calculator" is not defined.  

Done.

How can I fix? 
thanks