Re: can't update tex live

Hi Ann,

    I am trying to update my version of Tex Live (perhaps installed in
    2017 or 2018) on my iMAC (2017).

It might not be possible to update from that long ago. Installing MacTeX
from scratch, as Dick suggested, might be less trouble overall.

    /Library/TeX/texbin/tlmgr: only MacOSX is supported, not darwin 13.7
    (from sw_vers -productVersion: 13.7.8)

However, if you want to keep trying the upgrade, I think you can get
past that particular error by changing line 322 (or thereabouts) in the
file tlpkg/TeXLive/TLUtils.pm from this:
   if ($os_major != 10) {
to this:
   if ($os_major < 10) {

I'm not sure of the exact path to TLUtils.pm on your Mac, but I expect
it's somewhere under /Library/TeX. Running the command
  find /Library/TeX/ -name TLUtils.pm -print
will hopefully show the full path.

Hope this helps,
Karl