Your robots.txt and sitemap.xml files look normal.
Can you kindly share the Post URLS for which Search console is returning a 500 error?
Just to assure you 500 errors are related only to your blog server and this must be a temporary error caused when Google bot is not able to fetech the data it needs. It wont effect your blog's SERP ranking though.
Since it is just a single warning on few of your URLS, there is nothing to worry. Try resubmitting sitemap and recheck after 48 hours.