Q.Why Do I Have Hard White Discs On My Plum Tree
Tree 6 years old I have only ever had 2 plums. Hard shrivelled discs after flowers

Certified GKH Gardening Expert
Unfortunately, your photos did not come through. I am unable to see the damage. This can happen for many reasons, from nutrient deficiencies to pH imbalances. Insects can be another cause of deformed flowers or buds.
In the meantime, this article will offer some suggestions on the issue:
https://www.gardeningknowhow.com/edible/fruits/plum/plum-trees-not-fruiting.htm