Resolving sagetex.VersionError
This section is added to my initial answer, after I just ran into the same problem.
I solved the problem following the instructions on this page:
http://doc.sagemath.org/html/en/tutorial/sagetex.html
although for me with Sage 7.4 the location of sagetex was not at
SAGE_ROOT/local/share/texmf/tex/generic/sagetex
but at
SAGE_ROOT/local/share/texmf/tex/latex/sagetex
I copied that sagetex folder from my Sage installation to
~/Library/texmf/tex/latex/sagetex
(you may need to create some of the intermediate folders).
After doing that, everything runs fine.
Background on TeXLive on Mac OS X / OS X / macOS
(This is my initial answer which just gives related information.)
Under OS X 10.11 El Capitan and macOS 10.12 Sierra,
the MacTeX installer installs TeXLive in /Library/TeX/texbin,
while up to OS X 10.10 Yosemite, it would live in /usr/texbin.
More info about that in https://tug.org/mactex/UpdatingForElC...
Do you still have a /usr/texbin directory?
At some point the TeXLive distribution shipped a copy of SageTeX.
Maybe you have an old TeX installation either via MacTeX or via Fink, Homebrew or MacPorts?
You could check your PATH (echo $PATH) and see if there is any place where a sagetex.sty might be found.
You could try texdoc sagetex to see if an old SageTeX documentation is found.
Wow, that is an old sty file. You may have to remove some old versions first - I assume you looked through the instructions for how to do this since your report indicates you used the texmf directory.