After setting up a new Search Center, I tried to add the Refinement Panel web part, but was unable to locate it.
Navigate to your Site Collection and enable Search Server Web Parts.
To activate the Search Server Web Part feature
To open the Site Settings page for the top-level (root) site of the upgraded site collection, append /_layouts/settings.aspx to the root site’s URL, as follows:
In the Site Collection Administration section of the Site Settings page, click Site collection features.
For Search Server Web Parts, click Activate.
Link for more detail: