I\'m facing an issue that I can\'t find a solution for it. I have Python 3.9.0a1 and pip 20.0.2 and I can\'t install different packages like Pillow while with other package
As @Njord mentioned, PIL is not maintained. Its fork, Pillow, is. Here's information about the project. The fork is not yet updated for Python 3.9 - as many packages will not be. You'll need to use Python 3.8 in the meantime until the update comes.
Pillow will get Python 3.9 support in its 8.0.0 release. As of today, it's scheduled for October 15, 2020.
The reason this is happening is because PIL hasn't updated to Python 3.9 yet, I am not aware of any fixes but right now it is best to downgrade to a Python version that can run PIL.
You aren't alone in this problem.