Mac OS question - can't upgrade OSX for Da Vinci

I have made the big plunge and dumped Adobe Premiere.

I have 2 MAC Pro's and 1 MacBook Pro. They were all running OSX 10.10.5 and Da Vinci Resolve 12.5.
Six months ago I upgraded one Mac Pro to 10.11.5 and all was OK. I couldn't do the others at the time as they were mid project.

I upgraded to DaVinci Resolve 15 and needed to I upgrade to 10.11.6. This worked OK for the 10.11.5 Mac Pro but the 10.10.5 Macs won't upgrade; an alert says I need to upgrade to 10.11 first; that is what I am trying to do!!! Is there a version of El Capitan 10.11? If so, where can I get it? Apple support can not help me and tell me my Mac's are too old but all 3 are the same vintage and one of them is working fine, but surely, it can't be THAT HARD!

Anyone share an insight into this enigma?

Baz

[Peter Chamberlain]

I cant help you with this OS issue but first.. have you exported your project database before trying the OS upgrade?

If not, i recommend uninstalling 15, put 14 back in and then make a backup of each of the databases.

Baz, any chance you can put the same system that you have on your mac pro on the left-behind macs ? If it's ok*, you could make a clone of your existing ElCap and restore this image (dmg) on you 2 others mac.
FYI, I just did it to make a dual boot on my mac pro, and I used SuperDuper as always, but other tools work too

*this means you'll have to back up personnal files first to restore them too on the newly upgraded macs

If you’ve upgraded once to El Capitan, then you should have it in you purchases on the App Store. Assuming the OS is compatible with the Mac Pro version you have, you should be able to download it from there.

Cheers,
Mike

Apple also provides a direct link to the El Capitan installer if it isn't available in the Purchased tab: https://support.apple.com/en-us/HT206886

And a list of supported hardware: https://support.apple.com/en-za/HT202888