Why does my salt chlorine generator say "low salt" when my test strip shows normal salt levels?

The cell is almost certainly coated in calcium scale, which messes with the conductivity reading. Pull the cell, inspect for white or crusty deposits, and clean it with a 1:4 mix of muriatic acid and water (or a dedicated cell cleaner). Soak for 10-15 minutes, rinse, and reinstall. Nine times out of ten that fixes it.