ncatura Said:
"how to downgrade back to catalina from big sur OS? I tried to open my Quickbooks 2016 but couldn't and could with catalina OS. Is there some way to get it to open without having to buy the newest version of Quickbooks?"
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Two Thoughts:
I. Restore or Reinstall:
A. Restore from a Backup:
Have you recently created a backup of your Mac prior to all of this? if so, restore from the backup. If not, then take this as a learning experience, knowing to always backup your Mac, prior making any major change to your device, such as an application installation.
Or:
B. Reinstall it
If you have no backup: The best way around this is to backup your Mac, erase all the volumes, and then install it anew. Once installed transfer the folders and applications. Refer to my User Tip: How to Reinstall the macOS Anew - No TBM Prior to Upgrade. The scenario might be a bit different, but if you follow it, you will see how to perform what I mentioned above. Hope it helps.
II. Elsewise, Just Upgrade:
Why make it hard? Just upgrade your software. In my opinion, I would just upgrade to the compatible version of QuickBooks. Backup, once installed.