flush_rewrite_rules should only be called on the plugin activation OR after you update or add rewrite rules programatically. If it does not work after verifying this, there is some other issue on your site. Make sure you are not calling flush_rewrite_rules in constructor or somewhere else on every page load.
Change Project permalink to custom URL
your missing a slash in your perma link structure it should be /%category%/%postname%
The error is related with the Hosting Server PHP Version. So if you are on PHP 7.3.XX try downgrading to 7.2.xx or 7.1.xx and see if that helps! P.S. I was facing the same issue and had to downgrade to PHP 7.1 to fix it!
Try to use function sanitize_title() – documentation, which converts a string into a slug when you check if it is already in the permalink, and when appending it to permalink: function custom_job_post_type_link( $post_id, $post ) { $permalink = $post->post_name; $companyname = $post->_company_name; if ( strpos( $permalink, sanitize_title($companyname) ) ) { // <– Here return; } … Read more
Check this article on how to create a static homepage in WordPress https://wordpress.org/support/article/creating-a-static-front-page/
How can I make a post sub link?
URL gives 404 error of my subfolder site when i change permalink
How can I rewrite the URL of my custom taxonomy to include the year?
If it only breaks when the excerpt is empty, then I would check for the excerpt before outputting it. <?php if ( ! empty( get_the_excerpt() ) : ?> // test for the excerpt <?php echo esc_html( get_the_excerpt() ); ?> // escape the excerpt in case there is html added <?php endif; ?>