Most likely your server is not windows, and the plugin is scripted to look in windows based directories ‘\’.
I had the same problem but managed to sort it out by editing the following file ‘wp-content/plugins/search-autocomplete/searchautocomplete.php’.
Find lines:
$globFilter = dirname(__FILE__).'\css\*\*.css';
foreach( glob($globFilter) as $stylesheet) {
$newSheet = str_replace(dirname(__FILE__).'\css', '', $stylesheet);
$newSheet = str_replace('\\', '/', $newSheet);
Replace with:
$globFilter = dirname(__FILE__).'/css/*/*.css';
foreach( glob($globFilter) as $stylesheet) {
$newSheet = str_replace(dirname(__FILE__).'/css', '', $stylesheet);
$newSheet = str_replace('//', '/', $newSheet);
Hope it helps.