Some security plugins can disable capability "edit_theme" for admin user 


For iThemes Security  - do following steps 


1. Go to Security -> Settings from your Dashboard sidebar.


2. Go to WordPress Tweaks.


3. Uncheck Disable File Editor.


4. Click the Save Settings button.





OR you can put following code to functions.php 


add_filter("woe_user_can_add_custom_php", "__return_true");