Getting error in error log continously

Dear All

I am continuously getting following error in my error log.
2020-10-18 12:25:51 - PHP Notice: Undefined index: permission in /home3/shopp1pd/public_html/system/library/cart/user.php on line 26
Further certain users have also reported issue in adding products to cart through their mobile.

Pls check and guide. My website www.shoppingcharger.com

Thanks

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