The new Sierra render times are killing me, so I used Time Machine and downgraded to El Capitan.
This was overnight. Tried to open a current FCP X project-denied.

So currently reinstalling Sierra, but I really dont wanna have to use it.
Does anyone know how to make Sierra projects/libraries function in El Capitan?

Did you upgrade FCPX as well? If so the projects will not open if the Time Machine backup restored the older FCPX. You might be able to open up a backup but I am not sure. If not the new FCPX might work on El Capitan.

My render times sped up with the upgrades on two different computers. Which Mac are you using?

Late 2013 trash can using Drobo Pro and Artiroc external RAID drives

A goggle search show a very small population experience my problem. I created a new user as suggested, no go, I trashed my prefs, no go, ran the latest Onyx, no go

Oh well I can go back to taking my ergo walks.

I have the same computer. No problems upgrading.