You may have issues being able to install that Legacy version of Java 1.6 if you didn't have it installed previously before updating to Catalina. For some reason it doesn't allow you to run the older version because it tells you that you have a newer version of Java already installed.
I have a script I used that circumvents this by disabling the installation limitations. It's quite simple to use, you just need to download that Java 1.6 from the link in the thread above then run the script in your Script Editor and then you should be able to install the Java 1.6 correctly. If you want to use it let me know and I can help out.
Script:
https://pastebin.com/J4bQ3jr2