No tabs after adding the shortcodes

Hi,

I added to the post the code exactly like on the Other Notes site:
https://www.ads-software.com/extend/plugins/wordpress-post-tabs/other_notes/

and have… only this text. No tabs widgets. I didn’t set anything in plugin settings. I have checked it with 2 themes and sites…
Do you have any hint what I am doing wrong?

Thank you,
Cornelius