What is a Keurig?
Keurig is a beverage brewing system for home and commercial use.
Keurig beverage varieties include hot and cold coffees, teas, cocoas, dairy-based beverages, lemonades, cider, and fruit-based drinks. Keurig has over 400 different varieties and over 60 brands of coffee and other beverages. The Keurig’s chief plea is how freaking simple it is. You just fill up the water tank, pop in a pod, press start, and you’ll have coffee in less than a minute. This means you won’t have to grind coffee beans, figure out how much coffee and water you actually need, or wait for a whole pot to brew. 
What to do when Keurig says to add water?
If Keurig is asking you to add water, and there is already adequate water in the reservoir, these are some troubleshooting options we can try out
1. Make sure the water reservoir is seated properly in the Keurig base. Remove it and replace it as necessary.
2. Remove, empty water, clean reservoir, and refill. Then place the unit back on base. Turn off and on again.
3. Use filtered or bottled water, not distilled to fill the reservoir.
4. If none of the above options work, you will need to descale your Keurig.
5. Also, make sure that the water reservoir is seated properly in the base.
6. Remove, empty, and thoroughly clean the reservoir. Then fill with fresh water and place it back in the base.
7. If after trying both of the above steps, you still see the message it may be time to descale your unit. It is a good idea to clean your Keurig 2.0 brewer on a regular basis and it solves many common issues.
8. If you still see the message, it is probably your water. Keurig 2.0 is very sensitive to the type of water used, specifically, it does not like distilled water. The sensor that detects the water level is electrical. Without going into a nerdy scientific description that I don’t even understand, that sensor requires a solid matter for electrical current to work. There are little or no solids in distilled water therefore, the sensors are not activated leading the machine to think the water reservoir is empty. You should use filtered or bottled (not spring) water with your Keurig 2.0 for optimal performance.
Nevertheless, using the correct water in your brewer is of topmost importance. It is imperative that you use filtered water not only for the best tasting coffee but to ensure the optimal permanency of your device.
Lastly, if there’s enough water in the water reservoir, the reservoir is placed properly, but still having “add water” issues, try cleaning the reservoir, thoroughly. Remove the water reservoir and discard the water. Hand wash it with mild soap and non-abrasive cloth and rinse. Check and clean the valve at the bottom of the tank and on the machine where the tank goes. The valve can sometimes get clogged as well and this can cause issues. Fill the tank again to the MAX fill line (use tap, bottled or filtered water, but do not use softened water) and return to the unit. Make sure to clean both exit needles as well.