I hope you are well today and thank you for your question.
You can try achieving this by using the following small plugin specially developed for you that contains some custom code.
After installing and activating the above plugin please go to your site permalink settings in the admin area and save the settings.
Please make sure to save permalink settings as described above and try using the below plugin removing the above shared.
Thank you for sharing the site details.
I can see you are using older version 2.0.3 of theme on your site so could you please try updating it to the latest version?
I can see the page http://tkparnas.info/portfolio/kukhni-vardek/ is working but not http://tkparnas.info/portfolio/
Can you please try temporary enabling WP_DEBUG mode? To do this, just go to wp-config.php include this:
You might already have a line for WP_DEBUG, so just make sure it’s set to true.
Let me know what errors if any you are getting on the portfolio page.