Are you sure you are unable to save? Try selecting the Disc icon in the right pane, which contains the individual Track entries, then click on the Save button. Does that succeed?
Per the Picard documentation, Basics: Matching files & saving, a red rectangular icon to the left of a Track entry in the right pane indicates that there is a poor match between the characteristics of the music file assigned to that Track, and the metadata MusicBrainz has for that track. The icon for a failed write is a “red error triangle”.
On my copy of Picard 1.4.1 on Mac OS X 10.11, a Track entry with a poor match has not only a red rectangular icon, but also a red background for the track entry. See “Stairway to Heaven” below. But where there is a good match, there is a green rectangle icon. See “Don’t Worry, Be Happy” below.
Maybe post a screenshot of what you are seeing?