The error will show if the 'Childcare weeks open' field is left empty or not within the range 1 to 38 when adding childcare details for under 5 childcare that is provided by the school.
To amend this go to Admin > Census > Spring Census > Childcare details (in the Census Specific Data).
In this area click edit for the entry for under 5 childcare, enter the week open and click update to save.