SharePoint online new experience issues with file library

Hi

In looking at your screen shot, I think the issue is that you are trying to delete the default document library (/Shared Documents/Forms/AllItems.aspx) which I don't think you can do in modern team sites (I can't remember if you were ever able to do it but from the conversation I think maybe so). I've just had a look at a MS Demo tenant and there is no option to delete the documents in the UI, but the delete option is available on any new document libraries I create.  When you think about it, it makes sense that you can't delete that library as Shared Documents is sometimes the default sharing destination when you save. When you add users to the site, the link in the welcome email is for Shared Documents.  Hope this helps.