This is an example of a participle that doubles as a noun, so it can be confusing. Finnish is full of these. If you have a linguistic streak, you can take a look at the declensions for "syötävä" here: https://en.wiktionary.org/wiki/sy%C3%B6t%C3%A4v%C3%A4
"Jotakin syötävää" is not a clearly defined thing.
Haluatko kahvin? = Do you want a (cup of) coffee? <-- clearly defined thing
Haluatko kahvia? = Do you want (some) coffee? <-- undefined amount
Object case is a problem in learning Finnish - it has to come up early because you need it even for fairly simple sentences but the rules are complex and there are exceptions. An actual Finnish teacher would be quite forgiving with object case errors until you're well into your studies. Unfortunately an algorithm is not as considerate :)