Hello and good evening to you.
The waste toner tank has a life span that may have been exhausted. That is why even after emptying it and putting it back into the printer you still get the error. That being the case, you will have to buy a new one. Refer to page 74 of your user manual for the part name of the waste toner container. In case you do not have the manual, the part name is CLT-W406. Then go ahead and order a new one from Samsung or check for it at your local computer store.