Change the input format in drupal 8
Thanks to No Sssweat. His answer works. But I prefer to use sql so here it is.

Using Phpmyadmin navigate to node__body table (or just search it) then in SQL tab, write this:

UPDATE `node__body` SET `body_format`='full_html' WHERE `body_format`='full_html1';


Please notice the following:


in my case, I use different body format in my website so it is not good to change all body_format to full_html. 
after the migration I got full_html1, but you may get different body_format.


After execution the SQL command, Clear Drupal cache