Screen reads no cartridge when there is a full cartridge in the printer

This is a pretty common item, usually for HP printers however I've also seen this with non-HP printers. They REALLY want you to use genuine HP or Dell ink/toner in their printers since they make lots of money on consumables. That being the case, there is a chip on most original (OEM) toner cartridges that tells the printer that its a genuine cartridge. If you purchase aftermarket/remanufactured/3rd party toner to save a few dollars, they frequently don't have these chips, however other than that they work just fine.
If you have purchased a Staples/Office Max or some other non Dell branded toner, that's likely what happened. My suggestion is to live with this minor annoyance and just wait until the print begins to get lighter in color. THAT will be your indication that its about time to change the toner.
If you get this same issue with a new Dell branded toner, then you may want to try to find this chip and use a pencil eraser to carefully clean the electrical contacts on the toner cartridge. That may help the printer read the cartridge chip easier.
Hope this helps! Best of luck!