Theme Check plugin error when I add ‘©’ [closed]

You need to make sure that you have the call back function to handle your sanitization.

$wp_customize->add_setting( 'understrap_footer_text', array(
        'default'           => 'Copyright © 2017' ,
        'type'              => 'theme_mod',
        'sanitize_callback' => 'sanitize_textarea_field',
        'capability'        => 'edit_theme_options',
    ) );

So you need a function like this

/**
 * [sanitize_html description]
 * @param  [string] $input [input from textarea]
 * @return [string]        [sanitize all input no tags]
 */
function sanitize_textarea_field( $input ) {
    return wp_strip_all_tags( $input );
}