search – 0% of the site has been indexed. There are 400 items left to index

You need to run cron. Because building the search engine can take time, Drupal does it on cron runs (the scheduling system).

First, to make sure everything is OK, run cron manually from /admin/config/system/cron to ensure that the search items get indexed.

Then set up cron to run automatically following the instructions in the documentation.