Constraint recipe
This error means arithmetic, not timetable quality: you asked for more lesson placements than the week can hold once you count periods per day, teaching weeks, teacher load, rooms, and unavailable time. No generator can place 45 lessons when only 40 valid slots exist. Recount demand and capacity in the source data before you change preferences.
Key takeaways
- The week has fewer valid slots than required lesson placements.
- Recount periods, weeks, lessons, teachers, and rooms.
- Imported duplicates often cause sudden overload.
- Fix source data before changing spread or gap preferences.
What the error actually means
The timetable system is telling you that demand exceeds supply. That may be too many lessons in the period plan, too few periods in the school day, too much teacher unavailability, or too few rooms for groups that need to run at the same time.
This is not fixed by running the generator longer or raising a quality preference.
Where planners usually find the mismatch
Start with a simple count: required lessons per group or teacher versus available periods in the cycle. Then check whether specialist lessons need rooms that only exist in small numbers.
The validation before solver guide covers static checks and feasibility tests to run before you trust an automatic solve.
What to do next
- Recount periods per day, cycle length, and teaching weeks.
- Total required lessons per class, teacher, and room pool.
- Compare those totals with available periods after unavailable time is removed.
- Search for duplicate rows in imports or copied curriculum lines.
- Reduce lessons, add rooms, extend the school day, or rebalance staffing.
- Run validation again before touching spread, gaps, or other preferences.
Common mistakes
- Raising generator time limits when the count is already impossible
- Missing duplicated lessons imported from a spreadsheet
- Counting unavailable periods as if they were teachable slots
In Smootables
Validation Errors
Fix errors before generating timetable
- Teacher "J. Rivera" has 28 hours/week assigned; maximum is 24 hours/week.
- This period needs at least 3 room(s) for your student groups, but only 2 are available.
When required lessons exceed what your week can hold, Smootables surfaces a capacity problem instead of returning a broken grid. On the period screen, Validation Errors lists issues you must fix before Generate timetable runs, such as a teacher above their weekly maximum hours or not enough rooms for groups that would teach at the same time.
If those checks pass but the plan still cannot fit, the failure report names the shortfall: too few teaching slots on the school-day grid, too few rooms, or both.
- Trim duplicate placements or reduce lessons in the period plan.
- Add rooms under Resources or extend the school-day schedule if simultaneous groups need more capacity.
- Generate again only after Validation Errors clears.
Quick answers
Can a better generator fix too many lessons?
No. If valid slots are fewer than required lessons, the count must change first.
Should I relax soft constraints first?
No. Spread and gap preferences do not create new teaching periods. Fix demand and capacity first.