Making the Posts Page work

what theme are you using?

temporarily switch to the default theme Twenty Fourteen to see if you are able to set the posts page there.

if not, temporarily deactivate all plugins to see if that helps. if yes, reactivate one plugin at a time and test inbetween to locate the interfering plugin.

also, enable DEBUG to see if you get any error messages – http://codex.wordpress.org/Debugging_in_WordPress