What add-ons are you trying to cancel?  I’m not too familiar with add-ons on prepaid. Not even sure add-ons exist on prepaid. 

Could you recheck to make sure you were a prepaid and not a postpaid customer? I asked because the "manage add-ons" were only available for postpaid customer self-serve. Prepaid customer normally had booster to manage instead.

I want t cancel the U.S. Roaming Data Booster and U.S. Roaming talk Booster

Yes, I am cancelling for my prepaid account. Is there a way to cancel the boosters?

From the FAQ : A booster can’t be removed. It’ll be rolled over until it's completely used up.

More info here : https://www.koodomobile.com/en/help/prepaid-boosters

If you wanted to remove a booster, you wouldn't be able to do so. Prepaid booster is not refundable unfortunately, I  https://www.koodomobile.com/en/help/prepaid-boosters

I thought it is billed monthly! Thank you!