After you install one of the following Microsoft SharePoint Server updates, you can no longer delete list item attachments in the item detail panel:
This issue occurs because of changes that were made in how a lookup item is displayed in the item detail panel.
To delete list item attachments, follow these steps:
At the left-bottom corner of the list page, select Return to classic SharePoint.
Select the item from which you want to delete the attachments.
Select Edit Item.
Next to each attachment that you want to delete, select Delete, and then save the page.