How to solve unclosed tags in wordpress themes ?

there is no easy or general way;
https://codex.www.ads-software.com/Validating_a_Website

have you edited the theme?
if so, which templates?
possibly try to (temporarily) replace each edited template (one-at-a-time) with unedited ones, and check the validation inbetween;

as this seems to be with all single posts, start by checking single.php, and follow all ‘get_tempalte_part()’ or function calls in that template to try and find the error.

because someone familiar with your theme might be quicker to spot the error, try to contact the theme’s developer for support.

this forum’s support for commercial themes is very limited – https://codex.www.ads-software.com/Forum_Welcome#Commercial_Products