The issue looks like you have an “unexpected” value in the existing “coordinates” column of organisationunit table.
You will get the same error if you execute the following statement
select ST_SetSRID(ST_GeomFromGeoJSON('{"type":"MultiPolygon", "coordinates":' || coordinates || '}'), 4326) from organisationunit where featuretype in ('POLYGON', 'MULTI_POLYGON');
My guess is, there are rows in the organisationunit table which has featuretype column value as POLYGON or MULTI_POLYGON, but the corresponding coordinates column is maybe null (or not a valid geojson format).