I'm looking to create a new character with the Character builder, but there seems to be no option to choose Kenku, does anyone have any idea why?

You need to unlock the other non-free Races with a purchase from the Marketplace.

D&D Beyond can be used for free and contains information from the following "free to use" sources published by Wizards of the Coast:  D&D 5th edition SRD, D&D 5th edition Basic Rules, and Elemental Evil Companion.

Content from other sources, such as the Player's Handbook, Dungeon Master's Guide, or Monster Manual, will need to be purchased from the Marketplace or accessed via another user sharing content with you via their Master Tier Subscription.

 You find Kenku in "VOLO'S GUIDE TO MONSTERS".

You can unlock Kenku for $1.99

https://www.dndbeyond.com/marketplace/source/15

I have volo's guide but no option for kenku still

Did you purchase the whole book ($29.99) or just the compendium ($19.99)? The compendium allows you to read the book but not use it in the character builder.

If you did purchase the whole book, make sure your entitlements are in sync -- go to this page, and make sure it says that your entitlement data is in sync under 'Account Entitlements.' You can also confirm under Current Product Licenses that you have access to Volo's on this account -- if you don't, you might accidentally have a second login without realizing.

can we buy the race or do we have to buy the book

you can buy the race all by itself

So... I did that and it still isn’t showing up. Am I just stupid?

You're not stupid.  And this should be made clearer.