In class we did an experiment to measure chloride in different water sources, including a lake. We used Ion-selective electrode of Ag/AgCl technique. We did the calibration curve and all steps and measurments in different sources of water are consistent, for example drinking water was a bit below 250ppm. But when measuring lake water we get a measurment of 0, why could this be happening? I can't find a reason
Most likely this was an artificial "lake" sample and someone gave you distilled water instead. Making artificial water samples is not an unusual practice in teaching laboratories.
Natural water cannot have zero concentration of chloride ion. Sodium and chloride ions are very common contaminants even in pure reagents. There is no "zero" concentration concept in analytical chemistry at all. The proper practice is to write "Not detected" instead of saying 0 ppm. All it means that it was below the detection limit of the ion-selective electrode. Ion selective electrode usually measure samples in parts per million (mg/L) range. Typical limits of detections are in single digit ppm.