Time Machine backup drive doesn't have enough room to back up to
Time Machine stores multiple versions of file which change over time so if the backup drive has the same size as the source drive this situation does happen after some time (it also may happen if the backup drive is bigger but much more seldomly). At this point you have two options


swap your backup drive for a bigger one and start a new backup on that
accept the fact that older versions of your backup will get deleted by Time Machine