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.