• "You need to fill the start and end date of the break!" – every break row needs both a Start and End time.
• "The end time can't be lower than the start time" – the break's End must be after its Start.
• "This employee already have a break for this date/time" – the break overlaps another break on the same timecard.
• "Invalid Break Time!" – the selected Rule no longer exists; pick another rule (rules come from HR Tools > HR > Settings).

When any break fails, the whole timecard save is cancelled, so fix the row and save again.