I would like to ask about magento2 testing framework errors

I entered the following command. but error...

cd /dev/tests/functional

vendor/bin/phpunit --filter CreateCategoryEntityTest

PHPUnit 6.5.14 by Sebastian Bergmann and contributors.

Runtime: PHP 7.1.26 Configuration: /var/www/james/dev/tests/functional/phpunit.xml

PHP Fatal error: Uncaught PHPUnit_Extensions_Selenium2TestCase_WebDriverException: invalid argument: can't kill an exited process Build info: version: '3.141.59', revision: 'e82be7d358', time: '2018-11-14T08:25:53' System info: host: 'localhost.localdomain', ip: '127.0.0.1', os.name: 'Linux', os.arch: 'amd64', os.version: '3.10.0-957.5.1.el7.x86_64', java.version: '1.8.0_191' Driver info: driver.version: unknown remote stacktrace: in /var/www/james/dev/tests/functional/vendor/phpunit/phpunit-selenium/PHPUnit/Extensions/Selenium2TestCase/Driver.php:165 Stack trace:

0 /var/www/james/dev/tests/functional/vendor/phpunit/phpunit-selenium/PHPUnit/Extensions/Selenium2TestCase/Driver.php(71): PHPUnit_Extensions_Selenium2TestCase_Driver->curl('POST', Object(PHPUnit_Extensions_Selenium2TestCase_URL), Array)

1 /var/www/james/dev/tests/functional/vendor/phpunit/phpunit-selenium/PHPUnit/Extensions/Selenium2TestCase/SessionStrategy/Isolated.php(67): PHPUnit_Extensions_Selenium2TestCase_Driver->startSession(Array, Object(PHPUnit_Extensio in /var/www/james/dev/tests/functional/vendor/phpunit/phpunit-selenium/PHPUnit

/Extensions/Selenium2TestCase/Driver.php on line 165

Why do I get an error?