10.11.6 should be alright for running Band-in-a-Box 2020. Please check that you have the latest build update installed. Go to Help | About Band-in-a-Box and the build number is the number in brackets after the version year. The latest currently available for version 2020 is build 426: https://www.pgmusic.com/support_macintoshupdates.htm#2020_426

If updating doesn't help, please go to Options | Return to Factory Settings.


Best,
Chantelle