You know the one that says, "Unable to copy elements from file." It is the same window that only provides a cancel button reinforcing the software's claim that this Error cannot be ignored.
Have no fear! All is not lost. The solution to this problem is to simply open the other template file into the current session of Revit. Once open, you can use the Insert View command to bring those sheets/views into your current project or template with seeing the image shown above. Don't forget to change the file type selection to "All Files". Hope you found this helpful!