Your requirements could not be resolved to an installable set of packages.
Problem 1
- phpunit/phpunit[9.6.0, ..., 9.6.23] require php >=7.3 -> your php version (5.6.40) does not satisfy that requirement.
- Root composer.json requires phpunit/phpunit ^9.6 -> satisfiable by phpunit/phpunit[9.6.0, ..., 9.6.23].