You can't delete the copy because it is associated to the Seattle.html file.
What you have to do is create a copy of the Seattle.html file to. This will create a copy of the html file (Seattle_copy(1).html) that you need to rename to anything you want, maybe "delete" as an example. Doing this will give you a dialog that asks if you would like to update the hyperlinks. Select Yes.
This will rename the html file and create a new master page with the same name (delete.html & delete.master), and now you should be able to delete the first copy of Seattle.master that you created. To get rid of the extra html file and master page, delete the html file and the related master page should be deleted as well.
The steps is also covered here. Issue with creating a copy of Seattle master page - Mikael Svensson