The equiavlent variable name for our pro plugin is _sf_s.

For some users, that works. Otherwise, the coding will be to change what you have by first fetching the search stong, then echoing. But, try changing %s to %_sf_s and let me know?

You will need to replace 1234 with your form ID.

<?php
global $searchandfilter;
$sf_current_query = $searchandfilter->get(1234)->current_query();
$search_term = $sf_current_query->get_search_term();
?>
<header class="searсh-header">
<h1 class="page-title">Résultats de la recherche : <span><?php echo $search_term;?></span></h1></header>

See if that works?

Ah, sorry, I just realized, you want the category to show, NOT the search term?

That may be a little more complex. This MIGHT work:

<?php
global $searchandfilter;
$sf_current_query = $searchandfilter->get(1234)->current_query();
$search_cat = $sf_current_query->get_field_string("_sft_category");
?>
<header class="searсh-header">
<h1 class="page-title">Résultats de la recherche : <span><?php echo $search_cat;?></span></h1></header>

More complex is needed then.

<?php
global $searchandfilter;
$sf_current_query = $searchandfilter->get(1234)->current_query()->get_array();
$search_cat = $sf_current_query['_sft_category']['active_terms'][0]['name'];
?>
<header class="searсh-header">
<h1 class="page-title">Résultats de la recherche : <span><?php echo $search_cat;?></span></h1></header>