I am in the process of upgrading from PSE11 to PSE 2020. I have a catalog of 39K+ photos tagged with over 1,000 tags. I want to have the same tag orgnization in PSE 2020 that I had in PSE 11. I hoped to be able to open the PSE 11 catalog in PSE 2020 to do that, but PSE 2020 apparently does not recognize the PSE 11 catalog even though they are in the same Elements Organizer/Catalogs ProgramData directory.
Your Elements catalog needs to be converted from PSE 11 to PSE 2020. You may have missed the opportunity to do this when you first installed and launched the new version. Try going to File>Manage Catalogs and click the Convert button. A dialog will appear that will allow you to navigate to your old catalog and convert it to the Elements 2020 format. A copy of your old catalog will be created and you will still be able to use it in Elements 11.