I'm using the Pixelmon Modpack from Curseforge, version 8.1.2. I've been playing a single player survival world on peaceful for a bit and realized that there's no legandary spawns. I checked the config file to see if legendary spawns and global messages that announce the spawns were set to "true" and they were.
Pack: The Pixelmon Modpack by: PixelmonMod

I went into a creative world and used /pokespawn to summon legendaries like Articuno and Zekrom and it worked. I also tried /checkspawns legendary and it said there would be a spawn in 19min with 30% chance, meaning that it should be working right?

Mods in pack:
fancymenu_1.5.2_MC_1.12-1.12.2
konkrete_1.0.3_MC_1.12-1.12.2
BiomesOPlenty-1.12.2-7.0.1.2444-universal
Gameshark-1.12.2-6.0.4-universal
journeymap-1.12.2-5.7.1
NaturesCompass-1.12.2-1.8.5
BOP-Patch-1.0
Pixelmon-1.12.2-8.1.2-universal
PixelExtras-1.12.2-2.5.10-universal
OptiFine_1.12.2_HD_U_E3

Version of modpack: 8.1.2.

MC Version: 1.12.2

Forge Version: 14.23.5.2854

This is going to be down to bad luck, or being in an area that only has spawns in specific time slots that you're not hitting. Check this page for the details: https://pixelmonmod.com/wiki/Legendary_Pokemon

Check this page for an explanation on what times are when: https://pixelmonmod.com/wiki/Spawn_times

Keep in mind that spawns done through "/pokespawn" will not fire the event, and as such will not show the message. If you want to test things further, repeating "/legendaryspawn" a few times in different areas should eventually result in a spawn. This command does fire the event, though it has the same odds of working as a natural spawn attempt. (e.g. it fails often)

Closing this ticket as spawns most certainly work. Thanks for the report, though!

Side note: I would recommend updating Optifine to the current newest version for 1.12.2, G5. E3 will cause issues on your Forge build.