Your syntax is wrong. You have a closing PHP tag where there is no opening PHP tag at the end of the script. This means you are creating output before the headers are being sent.
Change this:
function prodmanager_html_page() {
?>
<div>
<h2>Product Manager</h2>
<!--usual HTML and PHP Code-->
</div>
?>
To this:
function prodmanager_html_page() {
?>
<div>
<h2>Product Manager</h2>
<!--usual HTML and PHP Code-->
</div>
<?php
}
You do not need a closing PHP tag at the end of your script. This is a good practice to help safeguard against stray whitespace after the closing tag which will give WP a white screen of death.
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