Webmasters tool will show that certain WordPress core folders are blocked by robots txt...it is normal for Wordpress sites.
If WP-ADMIN, WP-INCLUDES etc. are blocked by robot.txt, it’s NOT a problem because it has to be blocked by default, or else unwanted pages will get indexed in Google.
We only recommend change robots if there is any other folder than wp-admin that's disabled.
If all pages are blocked by robot.txt, you need to delete your existing robots.txt from your hosting.
Then set up a new robots.txt with the following plugin on your site
Once you have created a new robots.txt file, please resubmit to Google.
Your existing robots txt file should be located in the wp-content/plugins directory. To delete that, log in to your hosting account and navigate through the file manager to find the plugins folder. After deleting, download the plugin from https://wordpress.org/plugins/wp-robots-txt/
Follow the instructions here: https://wordpress.org/plugins/wp-robots-txt/#installation