Translation:The important thing is not what you do but how you do it.
The "thing" comes from the fact that "O importante" is a noun, not an adective. But the meaning is the same.
English doens't seem to have a noun "the important". Then you complete with other things like "the important one", "the important thing".
Adjective version: "O que você faz não é importante, mas sim o modo que o faz". (But this could somehow offend the listener, because you say directly that his work is not important)
Another: "Não é importante o que você faz, mas...."
This question highlights a weakness in duolingo. Although you can completely understand an expression like this and can translate it accurately, matching the exact words used in the 'correct' translation is a lottery. The focus of attention then becomes the limitations of the software. You might consider an override button, where a user can say 'I know this is correct'. Might mess up your stats a bit but will deliver a less frustrating user experience.