@chrischriskurisu Thank you for your feedback. As @matt_in_mito mentioned, the goal of Bunpro is to use previously studied grammar in sentences so that they both build off of what you have already learned and introduce you to new contexts/situations where that particular grammar point can be used.
The casual volitional is introduced in Lesson 4, while てしまう does not appear until Lesson 8, so unless you are adding entire lessons to your reviews daily, this should give you plenty of time to familiarize yourself with the volitional and how it is used.
Before you type your answer, the hint “[casual + volitional]” is shown in the answer blank (in the Japanese sentence). If you have all of your hints turned on, you should also be seeing "[casual・volitional (can also be translated as “let’s”)].
If you type in the plain form (dictionary form) or the polite ます-form, you will get the hint “Think ‘will’ (volitional).”
We try our best to guide you along if you happen to forget a certain form, rather than marking you wrong outright. We are always trying to improve how the system works by making it more intuitive and your feedback helps us do that. Cheers!