I'm having the same issue - was going to try and upgrade to El Captain from 10.10, but it seems as yours won't sync, that won't make any difference.
I won't lose much, but it is annoying as Evernote was a key part of my workflow.
Would you mind letting me know which version of Evernote you are able to run on your computer? Highest I can get to at the moment is 6.12.3
Think I'll just need to find a Evernote alternative until I can justify a replacement computer. The one I have now works really well, so I'm reluctant to spend thousands just for one App