Drupal - Search api excerpt are empty in the result

First go to this path admin/config/search/search-api/index/default_index/processors#edit-processors-highlight-settings make sure this settings are enabled

Next i created a view Global search which has search Excerpt field.

Finally i was able to see the highlighted text word after searching

Actually this is not default search form that comes in drupal8 it is exposed form in block if you see in view exposed form setting i have enabled it. once this is done, exposed form comes in block just place that in which ever regions you want.