I need someone to teach me how to fix this issue on my website when it happens 126 incorrect key file table

Hi there I need someone to teach me how to fix this issue and where i can find it from my website or server or back end of my site. error as follows 126 Incorrect key file for table './gadgetci_shop/[login to view URL]'; try to repair it in: [delete from whos_online where time_last_click < '1559909795'] I need a step by step instruction