You cannot to an “IN” query with an ACF select field. ACF saves this value as a serialized array (A string).
$posts = get_posts(array(
'posts_per_page' => 1000,
'post_type' => $post_type,
// 'paged' => $paged,
'meta_query' => array(
'relation' => 'OR',
array(
'key' => 'activite',
'value' => 'value 1',
'compare' => 'LIKE'
),',
array(
'key' => 'activite',
'value' => 'value 2',
'compare' => 'LIKE'
),
)));