Edit or enter excerpt text manually
Excerpt text is an intro text of your posts and it's being generated by the WordPress automatically from the first couple of sentences of your posts.
However, you can control or enter this text manually if you want from the post edit/add page.
Block Editor (Gutenberg):
once you are on the post editor page click on the Screen Options button located in the top right corner of your editor window and the modal box with section will appear in front of you. Now just activate the section “ Excerpts”. See image: