The object manager for unit tests is in magento/framework/TestFramework/Unit/Helper/ObjectManager.php and you obviously instantiated it, otherwise you would not get this error message. But you are using it wrong, it does not have a create() method. As explained in the linked answer, you need getObject():
public function getObject($className, array $arguments = [])
I am using the ObjectManager::create method to get an Instance of my Helper class in another custom class. So i shouldnt use ~::create at all?
Not with this object manager helper for unit tests. It has a different interface than the regular object manager. It sounds like you are passing the unit test object manager to a class under test. This is not how it is intended to be used.
In an integration test you can use: Magento\TestFramework\ObjectManager::getInstance() which works like the "real" object manager. In a unit test, you could instantiate or mock the real object manager Magento\Framework\App\ObjectManager.
But the fact that you need it, tells you that there's something wrong. Don't use the object manager to get an instance of your helper. Instead, add your helper as a constructor parameter and let Magento handle the Dependency Injection.
