Add the grapefruit along with the water into the inner vessel of your Instant pot.
Pressure cook it on high for 10 minutes.
Once the pressure has released naturally and the pin dropped open the Instant pot lid.
Add the sugar into the cooked grapefruit mixture.
Add nasturtium flowers into the cooked grapefruit mixture.
Switch on the Instant pot for 30 minutes on Saute mode.
The mixture after 10 minutes will start coming up, i.e. once the sugar starts boiling. At that point mix again and it should settle down. You would need to keep on doing this. Careful not to burn your hand.
Keep an eye after 20 minutes.
You can check if the marmalade is done using a wrinkle test on a cold plate. Once the marmalade seems to be done, add a drop or teaspoon to the plate. Wait for a min and then press it against your fingers. If the surface wrinkles it means it is done.
Alternatively, you can use a candy thermometer. Once the temperature reaches 104.5 deg c or 220 deg F you are done.
Switch off. Do not let it go over 220 Deg F or the jam or marmalade will ruin it.
Add cinnamon and give a quick mix.
Once done, carefully remove the inner of the Instant pot with the help of hand gloves and let it cool completely before filling it in jars.