Do you happen to run into this problem with a non-beta version of the Editor? Just want to ensure it’s not due to you using a beta version of the editor.
Post Processing 3.0.3 is no longer a core package (not shipped with the editor). The version you should use in 2021.1.0b6 is 3.0.2. In Package Manager Window, Click on the ‘+’ icon, then Add Package by name…: Name = com.unity.postprocessing and Version = 3.0.2. Then, Add