I'll try to guess.
You also have a bundle like liip/functional-test-bundle or other, which require PHPUnit in requirements (or even you require PHPUnit directly in your composer.json together with phpunit-bridge).
As a result you have two different version of PHPUnit installed in project, with two different api.
If installed liip/functional-test-bundle is your case and you do not want to remove it, you need to install by bridge same PHPUnit version as installed by Liip bundle dependency. You can set version through SYMFONY_PHPUNIT_VERSION env variable or directly in bin/phpunit file. Or you can redefine path to already installed PHPUnit version (in vendors/) through SYMFONY_PHPUNIT_DIR env or directly in bin/phpunit.
Note: you can not set SYMFONY_PHPUNIT_VERSION or SYMFONY_PHPUNIT_DIR through .env.* files, since bin/phpunit not read this files. So it should be real env vars.