phpunit Error: Class 'Prophecy\Prophet' not found

Question

I am in the process of writing some PHPUnit tests in Prophecy with the following code:

public function testItBuildsAndPersistsEnclosure()
{
   $em = $this->prophesize(EntityManagerInterface::class);
}

Which yields me the following output:

PHPUnit 6.5.14 by Sebastian Bergmann and contributors.

Testing Project Test Suite E.
2 / 2 (100%)

Time: 105 ms, Memory: 6.00MB

There was 1 error:

1) Tests\AppBundle\Service\EnclosureBuilderServiceProphecyTest::testItBuildsAndPersistsEnclosure Error: Class 'Prophecy\Prophet' not found

I'm a bit new to PHPUnit and testing in general but as I understand it, PHPUnit is supposed to have built-in support for using Prophecy which makes the error above really surprising.

I tried installing phpspec/prophecy but that process failed as shown below:

composer require phpspec/prophecy

Using version ^1.10 for phpspec/prophecy
./composer.json has been updated
Loading composer repositories with package information
Updating dependencies (including require-dev)
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - Conclusion: don't install phpunit/phpunit 9.0.1
    - Conclusion: remove doctrine/instantiator 1.1.0
    - Installation request for phpunit/phpunit ^9.0 -> satisfiable by phpunit/phpunit[9.0.0, 9.0.1].
    - Conclusion: don't install doctrine/instantiator 1.1.0
    - phpunit/phpunit 9.0.0 requires doctrine/instantiator ^1.2.0 -> satisfiable by doctrine/instantiator[1.2.0, 1.3.0].
    - Can only install one of: doctrine/instantiator[1.2.0, 1.1.0].
    - Can only install one of: doctrine/instantiator[1.3.0, 1.1.0].
    - Installation request for doctrine/instantiator (locked at 1.1.0) -> satisfiable by doctrine/instantiator[1.1.0].


Installation failed, reverting ./composer.json to its original content.