When version 28 became available, all attempts to install failed for MacOS version 10.11.16 on a 2009 Mac Pro. Safari version is 11.0.2. I uninstalled what I had, I made sure that no Flash files were left on my computer and did everything suggested on forums and from Adobe help website.
Finally, I reinstalled version 27 and that was successful and Flash now works again.
I hope this helps if anyone else runs into the same problem.
PS: I contacted the Adobe chat service and was just told to go look at the forums. Useless.