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
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