"Not Charging" on a Nokia device usually means that the charger deviates detectably from Nokia's standards. It might be defective, it might be a low-quality non-OEM charger, or it might be designed for another piece of equipment. I have had this message on Nokia phones and Tablet when I was trying various combinations to see whether I could reduce the number of chargers we had to pack for vacation.

There could be other reasons. As Tom61 says, your Tablet's charging circuit might have a flaw. Or you might have a substandard power supply in your house, delivering inadequate voltage to the charger. It's more likely to be the charger, though, and this is the easiest thing to test. You should be able to use any Nokia charger built for use in your country and having a 2 mm pin. (Older Nokia chargers have a fatter pin.)

If the problem seems to be inside your Tablet, there are other solutions. On eBay you can find charging cradles for any kind of Nokia battery. They come from Hong Kong, cost under $5 including delivery almost anywhere in the world, and work fine. You put your battery in the big rectangular space, plug your charger's pin into the small hole in the cradle, and plug the other end into the wall outlet as usual. (Some of them plug directly into the wall outlet without needing the charger.) There will be a light to indicate when your battery is fully charged: it will change colour, or stop flashing, or something like that, depending on the model you get.

Sometimes the charging cradle is offered with a spare non-OEM battery at an amazingly low price. This can work OK, but you will have to take care. I had one that was a tiny bit fatter than a genuine Nokia battery, and the battery door did not close easily over it. Before I got used to the extra care it needed, I pushed too hard and chipped a little bit of plastic off the Tablet. It was easy to repair with plastic cement, but I should have been more careful in the first place.