The error will show if the 'Childcare Week Open' field is left empty or not in the range of 1 to 15 when adding childcare details for holiday childcare.
To amend this go to Admin > Census > Spring Census 2020 > Childcare details (in the Spring 2020 Census Specific Data).
In this area click edit next to the holiday childcare entry to amend, enter the weeks open and then click update to save.