If you specify the index for the multiple checkbox in both your form and test, then it works. Form:

    <input id="department_1" name="departments[0]" type="checkbox" value="1">
    <input id="department_2" name="departments[1]" type="checkbox" value="2">

Unit test:

    public function testUserCreation()
        {
            $this->be(User::find(10));
    
            $this->visit('/users/create')
                ->type('First', 'first_name')
                ->type('Last', 'last_name')
                ->type('[email protected]', 'email')
                ->type('123456', 'password')
                ->type('123456', 'password_confirmation')
                ->check('departments[0]')
                ->press('Submit')
                ->seePageIs('/users');
        }

Using named indexes works as well.

    <input name="departments[department_1]" type="checkbox" value="1">
    // [...]
    $this->check('departments[department_1]');