Permalink problem: 404 error

Hello,
when I go to settings -> permalinks and choose the “plain” option, then all links work as expected, but when I choose something else the pages are not displayed. In the index.php of the theme I added this line <?php if ( !get_option('permalink_structure') ) { echo 'permalinks not enabled'; } ?>, but it didn’t produce any output.LoadModule rewrite_module modules/mod_rewrite.so is also activated and WordPress generates a new .htaccess file when I delete the old one. Nevertheless, I always receive a 404 error on all web pages, only the homepage on localhost/wordpress can be loaded. According to PHP info, the mod-rewrite module is loaded. How can I fix this error? Thanks for any help.