According to https://together.bunq.com/d/35022-haven-t-filed-your-taxes-de-fr-it-users-we-have-a-fix-for-you it should be active until end of June 2021. However it does seem that the code is not working.
Bunq support team, could you help out here, please?

Hi 👋 @Anthony-Blue-Dragon#217001

Please contact us via private support, and we’re happy to assist you further.

1. Go to Profile
2. Tap MENU
3. Select Get Private Support

Alternatively, you can send an email to support@bunq.com from the email address you've registered at bunq. More info on support here.