Translation:Could you repair my car as quickly as possible?
In the section's tips and notes you will find the definition. Kiel eble plej [kio ajn] is sort of an idiom to mean “as [whatever] as possible”. I think that we can try to sort out this expression. First we must remember that kiel is a correlative and that for correlatives the prefix ki- is used to indicate a interrogative or relative particle (a pronoun or a conjunction) and the suffix -el means “manner or degree”. Therefore kiel, when not used as interrogative meaning “how”, is a conjunction meaning “like”, “in the manner of”, “how” or even “however”. The rest is easier, in my opinion. Eble means “possibly” and plej means “most”.
We can translate Kiel eble plej [kio ajn] word-by-word: “In the manner possibly most [whatever]” or “In the manner most [whatever] possible”.
I remember in a previous skill, the verb kapabli was introduced. There were discussions about the difference between it and povi. While I agree with you that "are you able" and "could you" are often interchangeable in casual conversation, Duolingo can be very rigid at times. So when I see some form of "can", I opt for povi, and vice versa.
I also note that "are you able" is in the indicative, so the verb would end in -as, whereas povus is in the conditional. Duolingo would mark a translation as incorrect for that alone.
I don't envy professional translators who have to be very careful about these things!