Hello/Hi/Greetings,
  try to log in admin ---> tools ----> comments
(check all)

thank you

You can use phpMyAdmin, or whatever database admin frontend your hoster offers.

Backup your database!

Open the Piwigo database, locate table piwigo_comments (or with the corresponding prefix instead of piwigo_ if you used a different one), open it, let all rows display (or in bulks of 500 or so), select all, delete.

If you have access to command line SQL only, the command for the not faint of heart would be TRUNCATE TABLE piwigo_comments or if truncate is not available then DELETE FROM piwigo_comments

Do not delete "a field" nor the structure of the table, delete the records of the table piwigo_comments, that is you clear the table of all records, not the table itself nor any fields. You will lose all existing comments. New comments will be added to the table.