when click on 'Add to favorites' link

i am receiving this error message when click on ‘Add to favorites’

Fatal error: Call to undefined function wp_get_current_user() in /hermes/bosweb/web172/b1727/nf.kapturedbykathycom/public_html/wp-includes/capabilities.php on line 1188

then i try this fix

You open capablities.php and add require_once(‘pluggable.php’);

after this i am getting this error

‘Fatal error: Using $this when not in object context in /hermes/bosweb/web172/b1727/nf.kapturedbykathycom/public_html/wp-content/plugins/image-store/admin/settings.php on line 17’

please help me to fix.
my site is
http://www.kapturedbykathy.com/galleries/gallery-50/