This module requie specific versions of the silverstripe framework. If the version you are using is not listed as supported you will not be able to install the module.
- Silverstripe framework: 4.5.0
- PHP: >7.3
To install the module just executed
composer require botzkobg/silverstripe-phpunit9
The first thing to do is to update the version fo the phpunit in your composer.json file. Replace the version there with ^9.0.0.
In your tests you need to extend SapphireTestPhpUnit9 instead of SapphireTest and FunctionalTestPhpUnit9 instead of FunctionalTest.