You have a few options - and they all boil down to getting less light into the camera:

1. Smaller aperture using a smaller aperture (higher f-number) will make the image darker, it will also increase the depth of field (usually a good thing in landscape photography) and will reduce sharpness if you push it past a certain value (test with your own camera/lens combination to see where the softening get too bad for your taste).

2. Lower ISO but don't get into the "extended ISO" range (on Canon lower than 100, not sure about Nikon)

3. Time of day around sunrise and sunset it's darker outside so you can get longer shutter speed, also, the light is usually softer and more directional.

4. ND Filter an ND filter is basically "sunglasses for you camera" it cuts the amount of light without affecting colors and lets you increase exposure time without changing other parameters

5. Faster shutter speed this is last because you wanted a slow shutter speed, but if you can't use any of the other options you'll have to compromise on shutter speed.

You also have some options in post-processing, they are generally not as good as getting it right in-camera but are way better than nothing:

1. Reduce exposure in post If the image isn't too bright you can use software like Lightroom to reduce the exposure and recover the image.

2. Stacking Take multiple images with the longest shutter speed you can, then average them in software, this causes blur that is similar to the long exposure blur.