Make sure all your plugins and theme are up to date, compatible with 4.8.1
Flushing any caching plugins you might be running, as well as server and/or browser caches. Not just your browser, but any op cache or content network cache as well such as CloudFlare.
If that does not resolve the issue, try deactivating ALL plugins temporarily to see if this resolves the problem (plugin functions can interfere). If this works, re-activate them individually (one-by-one) to find the problematic plugin(s).
Or
Edit manually wp-config.php file.
Find the line
define('DISALLOW_FILE_EDIT', true);
(it is probably true by default installation)
and change in
define('DISALLOW_FILE_EDIT', false);
That might solve your issue. Hope this will helpful for you.