From Joomla! Documentation

If you have a message saying tags have not installed, it means that the full update sql did not run.

Updating to 3.1.1 may solve this issue.

If it does not or for some reason you cannot update to 3.1.1 the following steps will resolve it.

  1. In your file system go to /administration/components/com_admin/sql and find the file 3.1.0.sql. Open it in a plain text editor.
  2. Determine your database prefix by looking at the server tab in global configuration or by looking at your database table names in phpmyadmin or whatever system your host provides for managing your database. For example you may see xyz123_assets ... in this case xyz123 is your prefix.
  3. In the text editor search and replace #_ with your prefix. Note that the replace should only replace the first underscore ... so now you should have for example xyz123_content_types.
  4. In phpmyadmin go to the SQL tab and paste the full file from the editor (or do similar in another system).
  5. click Go (or do similar in another system)

At this point tags will be fully installed.