“Could not insert term into the database” when trying to add category

I’ve not been able to create new categories or menu suddenly and whenever I tried to do that returned a message “Could not insert term into the database.”

Google search one thread suggested DB optimization plugin but did not work. Another one (https://www.reddit.com/r/Wordpress/comments/5iroaa/could_not_insert_term_into_the_database/) advised 3 things to do:

1) go to phpmyadmin then in Structure make sure AI is checked. Mine was not checked so i checked it, then tried to create a new category but now returned a message, “some error occurred. Please reload the page.” I reloaded and the same message came back.

2) “stop playing in PhpMyAdmin and remove it from your server.” I have no idea what this is about.

3) in Operations, set the number for auto-increment to be higher than your highest term_id. After I set the number and click on Save, an error message showed up at the bottom,

“Notice in .\tbl_operations.php#155 Undefined index: row_format

Backtrace”

My site is still localhosted on my notebook. I reinstalled WP from scratch then imported the database and files but that did not help, either.

Hope all this makes some sense to someone here so can shed some light on my problem. Thank you.