Information

(you can remove whatever is not applicable)
Minecraft version: 1.18
Modloader: Forge
Forge version: 38.0.10
Environment: Singleplayer

Mod name: configurableextramobdrops
Mod version: 1.18.0

Question

What am I doing wrong?

'minecraft:enderman' : '{id:"minecraft:obsidian",Count:1b,tag:{dropchance:.5d}}}',

Is what I have added for giving enderman a 50% dropchance of 1 obsidian.

Are you using the in-game commands?

Using the in-game commands is the easiest to add extra drops. The correct syntax is:

'minecraft:enderman' : '{Count:1b,id:"minecraft:obsidian",tag:{dropchance:0.5d}}',

Command (while holding obsidian):
/cemd addhand enderman 0.5

Thanks for giving the mod a try! :)