Reasons for 'Red error report' when you close a Schedule

When you Close a Schedule you will sometimes be shown a Report which may contain Red, Blue and Green messages.

Possible Reasons:
The usual reason for the Red error messages is one (or more) of the following:

1.
A Class has been deleted from the Classes Screen after scheduling was started.
If you decide you no longer need a Class, simply leave it in the classes list; it does no harm.  Or if you want to make sure it doesn't appear on any of your timetable printouts, just give it a printing name of  *D  which denotes it as a "Dummy" class.

2.
Classes have been renamed on the Classes Screen after scheduling was started.

3.
Classes have been changed in the batches, for activities where one or more lessons had already been scheduled.
For example, suppose you had an activity "11A  SS  KJo" in the batch.  If you assigned the first "S", but then returned to the batch and changed the "11A" to "11B", then you would have a mismatch between the batch and the schedule; the schedule would have a lesson for 11A, but the batch would think that lesson should be for 11B.
If you really do need to make such a change to an activity, the only safe way forward is to first Un-assign any lessons for that activity, so they are off the schedule before you make the changes.
 
4.
Another thing to watch out for, is you shouldn't Delete or Insert any other basic data (teachers, rooms, subjects) once you have started scheduling, as you may be effectively 'changing the rules' part-way through scheduling.

Any of these can effectively 'change the rules' part-way through scheduling, and this can upset the schedule.
If you really do need to make widespread changes to your basic data, then you should start a New schedule afterwards.  That is the safe / clean way forward.
 

Solutions:
If you have some red messages:

a) either: Restore your most recent good Backup (if you have one)

b) or: you can ask TimeTabler to fix this problem by going to "Help -> Validate current schedule" (ie: the second-bottom item from the "Help" drop-down menu at the top of the opening screen), click "Close" then say "Yes" - you do want to fix the problem.  Then "Close" again.  This will tell the Batch that the problem lesson(s) is not scheduled, and place the lesson(s) back in the list of outstanding lessons awaiting scheduling - you can then schedule it to the period you want.