Widget does not get saved in the sidebar

Hello,
I was not able to use this plugin because every time I try to add the Facebook page plugin widget to my sidebar, it does not get saved. I go to Appearance –> Widgets, then I add the Facebook page plugin widget to the sidebar, then save, then leave the widget section. If I come back to the widget section, the page plugin widget is removed from my sidebar. Do you know why I get this issue?
Thank you very much in advance.

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