Post looked after arrangements indicator is missing or invalid.
This error will occur if a student is recorded with the now-inactive Post Looked After Status "Adopted from care (Inactive)".
In order to resolve this, head to the student's profile, and scroll down to the Looked After (In Care) section. You should see the Looked After (In Care) status is set as "Adopted from care (Inactive)".
Click on the Looked After (In Care) status field, then click Edit on the slideover that appears.
You'll need to change the selected value to either "Adopted from care inside England or Wales" or "Adopted from care outside England or Wales", depending on the student's circumstances, then click Save Changes.
Once this has been done, regenerating your census should clear error 1871.