That is a quite old version, and we have moved domains, which is probably why it won’t activate. The latest version is 2.4.5.

Can you update this first?

Please make a full site backup.

With such an old version, you should NOT delete the plugin from the plugins page in your WP Admin, as doing so will delete your forms also. This is not an issue with more recent versions.

You need to do the update manually (after downloading the latest version from your account area):

Manual installation instructions:

1) Deactivate S&F Pro in WP Admin Plugins page (do NOT delete the plugin from within WP Admin Plugins page; otherwise you will lose your search form)
2) Via FTP (or cpanel file manager if you have it), delete search-filter-pro folder from the wp-content\plugins folder on your server. If you wish to keep the folder, just in case, do NOT rename it, instead move it UP a folder into the wp-content folder.
3) Unzip the file that you have downloaded (if you are using cPanel File manager, you can upload the zip file and then extract it in the plugins folder).
4) Upload the search-filter-pro folder to the wp-content\plugins folder on your server.
5) Activate Search & Filter Pro in WP Admin Plugins page.

You should also re-build the Search & Filter cache also (edit any form and the button to rebuild the cache is in the meta panels at the side).