Beginner’s question about shortcodes

It looks like theme issue. Switch to default WordPress theme and check it. I have tested it on WordPress Twenty Seventeen theme.

For testing:

1) Add below code in theme functions.php

// Sample shortcode [testme]
function testme_cb( $atts ) {
    return 'Working? Maybe.';
}
add_shortcode( 'testme', 'testme_cb' );

// Enable shortcodes in text widgets
add_filter('widget_text','do_shortcode');

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2) Goto Appearance -> Widgets and add shortcode [testme] in Text Widget.

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3) Check it on frontend.