Let's play "identify the missing part"!
While riding, something which holds the fender to the frame between the chainstays and just behind the bottom bracket fell off:
Please help me make my bike stop making horrible rattling noises.
You need
For detailed instructions on metal fender install see the detailed Velo Oranges fender manual.
It looks to me like you just lost the nut. A stainless steel nut and washer or a stainless steel nylock nut would work best for this application. These options would help prevent the nut from coming loose and falling off again. You could also try using threadlock to help secure the nut as well.