Using FarmingForBlockheads_1.15.2-6.1.2.jar
Wither rose is also not in the market but that may be intended.
Also side note that when I tried to add Lily of the valley manually it ended up in the "other" tab, despite my setting the category to vanilla_flowers

Also the spawn eggs lacks panda and bee.

The category is called "farmingforblockheads:flowers" - this example worked for me:

"customEntries": [
    {
      "output": {
        "item": "minecraft:stick"
      },
      "category": "farmingforblockheads:flowers"
    }
  ]