I'd bet it's because TPM and Oekaki have separate member lists. That is, just because you're a member of TPM forums doesn't mean you're a member of Oekaki. I've never used it before, but I tried logging into the system and was unable. I've been a member for a long time, so it wouldn't be an issue of whether the system had been updated. You'll likely need to register for Oekaki at
this link in order to use it. Then, everything should work fine.
EDIT: And Asilynne, I checked the Oekaki memberlist, and there was no entry for Asilynne. Oddly, though, there was an entry for Lintzilla, in addition to a "Lintzilla ver.2"... odd.