A couple of days ago started receiving this error message when someone tries to go to our shopping cart. I'm am very much a novice, no idea what to do to fix this.  Thanks for any help you can give

warning: Undefined array key "rt" in /home2/agreenerway/public_html/shop/core/init.php on line 81warning: Undefined array key 0 in /home2/agreenerway/public_html/shop/core/lib/error.php on line 114error: Uncaught Error: Undefined constant "IS_ADMIN" in /home2/agreenerway/public_html/shop/core/lib/exceptions/exception.php:93 Stack trace: #0 /home2/agreenerway/public_html/shop/core/lib/exceptions.php(111): AException->showErrorPage() #1 /home2/agreenerway/public_html/shop/core/lib/exceptions.php(59): ac_exception_handler(Object(AException)) #2 /home2/agreenerway/public_html/shop/core/lib/error.php(114): ac_error_handler(2, 'Undefined array...', '/home2/agreener...', 114) #3 /home2/agreenerway/public_html/shop/core/lib/exceptions/exception.php(76): AError->toLog() #4 /home2/agreenerway/public_html/shop/core/lib/exceptions.php(109): AException->logError() #5 [internal function]: ac_exception_handler(Object(AException)) #6 {main} thrown in /home2/agreenerway/public_html/shop/core/lib/exceptions/exception.php on line 93