How to add “Read More…” link in twentytwenty

By going to the Theme Editor, you can add the following snippet at the end of the functions.php file:

  function twentytwentychild_excerpt_more_add_continue_reading( $more ) {
    return ' [...] <div class="read-more-button-wrap"><a href="' . get_permalink( get_the_ID() ) . '" class="more-link"><span class="faux-button">Continue reading</span> <span class="screen-reader-text">“' . get_the_title( get_the_ID() ) . '”</span></a></div>';
}
add_filter('excerpt_more', 'twentytwentychild_excerpt_more_add_continue_reading' );

Source: https://wordpress.org/support/topic/continue-reading-doesnt-appear/