Can't drag file into group libraries

Last semester, I did it easily, both draging files to "My library" and "Group libraries". After that, I haven't used Zotero for a while due to semester break. When I start using it again for preparing my term, I try to drag file from my computer to Zotero app and nothing happen. It seems like there is no response from the app.

I try to add it using "Add Attachment" >> "Attach stored copy of file", but the button is grey and I can't press that button.

What should I do?

Thank you in advance
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