if i want to say I don't know, will it be "sijui"?

Is there a specific reason "knowing" isn't accepted here?

HUGE NOTICE:

Thanks to Duolingo Tree Enhancer I have now TTS in my Swahili lessons. I'm so happy about it! If you also want to listen the pronunciation of Swahili words, you will really want to install it (it requires before a browser add-on [Tampermonkey in Chrome], but the instructions are in the DTE Github's page).