(Please start new threads for new issues — the thread you posted to is about dragging collections and isn't related to your question. I've moved this to a new thread.)
You can't drag items to My Library — all items in your library are already there.
If you explain exactly what you're trying to do, we can help.
Your answer to a question was 'drag it to mylibrary'. I tried dragging all the items within what I guess it is a subcollection (it has a folder image) into 'MyLibrary' and it didn't work. See image below. What I am trying to do is to move all items from inside the tiny folder thingy to MyLibrary, so I can erase the folder but not it's content. I thought that was 'moving a subcollection to a Collection'
No, that's about actually moving the collections, not the items in them. If you just want to get rid of a collection and leave items only in My Library, you can just use the "Delete Collection" option (which is above and distinct from "Delete collection and items" in the right-click menu)
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You can't drag items to My Library — all items in your library are already there.
If you explain exactly what you're trying to do, we can help.
What I am trying to do is to move all items from inside the tiny folder thingy to MyLibrary, so I can erase the folder but not it's content. I thought that was 'moving a subcollection to a Collection'
https://s3.amazonaws.com/zotero.org/images/forums/u5687390/7qwr6i5z32utr0tf0y5r.png
Also might be worth reading https://www.zotero.org/support/collections_and_tags#the_zotero_collections_model to make sure you understand how collections work in Zotero