Nikon lens not working?

Recently, I tried using my Diana Telephoto Lens. I have a Nikon adapter, so I was using that also. I put my camera onto manual, as the camera doesn't pick up the lens on auto. However, the pictures are coming out black, when I try to use the flash, it says 'Flash is in TTL mode. Choose another setting or use a CPU lens.'
How do I fix it so I can use my new lens?

Added (1). My Nikon is set to manual. I figured that part out, it's just not taking pictures with flash, however is without (I figured the lighting was too dark, as I tested it out with my other Nikon lenses and they were all the same, dark without flash but worked with)

Buy a Nikon lens. Throw that other one in the garbage.

If the Diana Lens has not go AF on it it won't work on Auto and if the Nikon Adapter has not got AF on it, they will only work on Manual. Both need to be AF to work on auto.

You need a manual flash or none at all. You see a CPU lens has electronics that help process all the data that the camera senses then the lens informs the camera what shutter speed/flash power etc to use. Since you're without a CPU in that adapted lens the camera doesn't know what to do, it doesn't have any information therefore it can't set a shutter speed or a flash intensity.

Therefore you'll need to use a suitable manual flashgun in order to get it to work properly, or use a tripod to get a slow enough exposure in manual mode without flash.

You really need to take at least one class in photography.

Your troubleshooting is sort of scattered.

If your Nikon is an entry level camera, then the light meter will NOT work with any lens but an AF, AF-I or AF-S lens. You will have to use an external light meter or guess at the exposure in order to get correctly exposed images.

The flash sync is probably 1/200th second. Make sure your shutter speed is at that speed or slower