notice: undefined variable: post_id in /home/

Error 👉 
Notice: Undefined variable: post_id in /home/u199577564/domains/freesarkarijobalert.in/public_html/wp-content/themes/FreeSarkariJobAlert.in-Theme/single.php on line 41

In my website every post I got this error message. How can I fix this issue please help me where & What coding I have to use. Here I providing you the coding which available in my website themes. 👇

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<?php
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get_header(); 
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?>
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<center>
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<style>
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table a{
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  color:blue;
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  font-weight:700;
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​
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}
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table h1{
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  color:#222222;
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font-weight:800;
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​
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}
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table h2{
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  color:;
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font-weight:700;
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​
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}
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table h3{
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  color:green;
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font-weight:700;
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​
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}
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table h4{
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  color:#ff00ff;
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font-weight:700;
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​
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}
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table b{
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  color:red;
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  font-weight:700;
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​
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}
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​
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</style>
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<div style="width:75%;height:auto;margin-top:0px;">
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<table style="width:100%;height:auto;">
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<tr>
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<td style="width:55%;"><h1><?php echo get_the_title( $post_id ); ?></h1><hr>
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​
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    <?php if ( have_posts() ) : ?>
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    <?php while ( have_posts() ) : the_post(); ?>
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    <div <?php post_class(); ?>>
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        <?php the_content(); ?>
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        <?php edit_post_link(); ?>
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        <?php wp_link_pages(); ?> 
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    </div>
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<?php endwhile; /* rewind or continue if all posts have been fetched */ ?>
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<?php else : ?>
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<?php endif; ?>
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<td style="width:20%;">Responsive Ads Here!</td>
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</tr>
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</table>
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    <div id="copy"> © Copyright 2018-2019 at <a href="<?php bloginfo('url'); ?>"><?php bloginfo('url'); ?></a>
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<br />For advertising in this website contact us <a href="https://freesarkarijobalert.in/contact-us">admin@freesarkarijobalert.in</a><br><br></div></br>
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</div>
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</center>
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<?php get_footer(); ?>
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​

• This topic was modified 6 years, 4 months ago by Jan Dembowski. Reason: Moved to Fixing WordPress, this is not an Everything else WordPress topic

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