How to alter column to make it primary key when one already exists mysql8?
If you would like to have other tables reference just the title field, then you can add a UNIQUE index.
First, edit the tours table to add a NOT NULL constraint to the title field:
ALTER TABLE `tours` MODIFY `title` VARCHAR(255) NOT NULL;

Now you can create the index:
CREATE UNIQUE INDEX `udx_title` ON `tours` (`title`);

Be sure to confirm that all values in title are unique before doing this. Once done, you will be able to have other tables reference the field.

Hat tip to Vérace for pointing out that title could be NULL, which we do not want.