The bike has threaded inserts that are pressed into the subframe where the seat bolts to. These can become loose with time. With one of mine loose, I could not bolt that side of the seat down. The insert would just spin in the subframe. I managed to fix it and here is what worked for me.
These threaded inserts are known as rivnuts or rivet nuts. I used my tap and die set to determine that the seat bolt for it was a m6 1.00 thread pitch bolt. I used that tap to clean up the threads on the spinning rivnut. Often these rivnuts are cross threaded as part of the problem. Once the rivnut has good threads again onto the actual work.
A metric rivnut tool and potentially m6X1.0 rivnuts are the speciality items needed.
I recommend steel (to be strong) ribbed (to resist rotation) rivnuts. Aluminum is an option as well but that will be susceptible to thread damage. Some local shops will have these parts. I bought a small amount off ebay.
As for a rivnut tool, there are various options. See what your local stores have or check online. There are all different types from pneumatic to hand tools. Some expensive, some less so. Here are a few examples of more expensive hand rivnut tools.
Being that I didn't know if this work work, I found an inexpensive hand rivnut tool on ebay and bought that one, from GB and all.
The loose rivnut on the subframe can either be replaced or repressed. In order to replace the rivnut it has to be drilled out. You want to drill out the rivnut with the smallest drill bit that will get the job done. I've read that a 9mm bit will work but search the internet for info. Also be aware that part of the rivnut will fall into the subframe and I don't know if it can be removed or how easily so. For these reasons I decided to repress the problem rivnut.
I used the instructions on the rivnut tool I bought with the M6X1.0 attachment and sure enough I was able to retighten the rivnut on the subframe. Success! Essentially, you screw the correct mandrell (M6) to the rivnut (I had to use a pair of pliers to prevent the rivnut from spinning as I screwed in the mandrell) and then screw the mandel onto the tool, then squeeze the tool handles to press the rivnut back onto the subframe. Be aware of the position you hold the tool when doing this so that you press the rivnut in a flush flat manner to the subframe. I did all this work with the subframe on the bike and essentially just the tail cowl off. I didn't go crazy pressing the tool together either. While it took a lot of force to squeeze the handles, I did not squeeze the handles completely together. It didn't need that to get the rivnut pressed back in.
Based on my experience I recommend trying to repress the original rivnuts before any more drastic action of replacing rivnuts.
Hope this helps.