Dusk now provides optional support for PHPUnit 8, which requires PHP >= 7.2 Please read through the entire list of changes in the PHPUnit 8 release announcement. Using PHPUnit 8 will require Laravel 5.8, which will be released at the end of February 2019.
You may also continue using PHPUnit 7, which requires a minimum of PHP 7.1.
Laravel 5.7 is now the minimum supported version of the framework and you should upgrade to continue using Dusk.
The setUp and tearDown methods now require the void return type. If you were overwriting these methods you should add it to the method signatures.