8th Anniversary Clean Slate Scroll 20%

Is it supposed to work only on the armor and not the weapons? I just got 1 to try and slate a soul shooter and got the message
"You cannot use that item on this equipment"
I was planning on making a couple of weapons +9 with 20%'s... I guess i just wasted all my meso on maple leaves

Depends on what your Soul Shooter is.
If you're talking about the purple ones, I'm pretty sure they only work on Anniversary gear.
If you're talking about the white ones, then I don't know what to say, as they should work on anything with slots.

I'm talking about the purple ones that say 8th anniversary, and yes I'm trying to use on an 8th anniversary soul shooter

It should work on weapons, but the thing is, (what I think happened) while we might have 8th Anniversary Soul Shooters, Shining Rods, Fans, and Katanas, when they released the 8th Anniversary Equip Only Clean Slates in kMS during their 9th Anniversary, those weapons, and the classes that use them, for that matter, didn't exist. The scroll won't work on anything that it isn't set to work on, sort of like an exception list, I suppose. Since those classes and weapons didn't exist at the time of the scroll's conception, they're not on the list, and while we have them, they weren't put on the list when we got the scroll. The 10% 8th Anniversary Weapon Only scrolls, as well as the 8th Anniversary Chaos Scrolls and Amethysian Polishing Feather, also lack the 8th Anniversary Soul Shooters, Shining Rods, Fans, and Katanas. I guess someone should test this out, though. It's a pretty small oversight, but understandable, I mean, I barely even know they, the Anniversary Item exclusive scrolls, exist.

Tested on a few other weapons and you're right, they don't work on the ones you mentioned, their success rate seems lower than 20% though or i'm just VERY unlucky since 2/23 have worked so far <_<