Mamiya645 afd I used Nyquist flash, and the flash came on, but the picture was very dark. Solution, how should I set up the camera?

You measure the flash data with an exposure meter, and then you will give an aperture value. Expose with aperture value1125s. I guess your aperture value is too high, or your shutter between mirrors is not working, or the reflector is not lifting. First remove the back with a low-speed shutter, and see if the shutter and reflector work normally from the back of the machine. If not, check the aperture value and fix it.

Did you ask a question before, mamia's flash is black?