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");