I understand you're looking for a manual for the product. However, "Gardena 01891" is not a complete model number for most Gardena devices (e.g., a pressure sprayer, pump, or watering timer). It might be a partial code, a part number, or a misreading.
To get you the correct , please check your device for a full Model No. or Art. No. (e.g., 1866-20, 9030, 1875). gardena 01891 manual