The license plates Illinois won't issue - from the state's own lookup and the records it has released.
Like every state, Illinois refuses plates it considers offensive, profane, sexual, hateful, drug or violence related, or that look official (like police plates). Reviewers also read plates backwards, with numbers as letters, and in other languages.
Most states let you ask for a review or appeal in writing. Check Illinois's DMV for how - and explain what your plate means, since reviewers often refuse plates they misread.
Use the rejection risk check in the VanityPlate dashboard. It looks for the things reviewers look for - hidden words, words read backwards or with numbers as letters, official-looking wording - and tells you if any state's own lookup has flagged the plate.