Translation:That house had few windows, and so it was not very bright.
"That house had few windows, and therefore wasn't light enough."? Could a native speaker say this? (it was deemed wrong, which I thought harsh)
FWIW The reason is that "enough" is not accepted. Light is accepted. I would think that "therefore wasn't light enough" would be closer to "nebyl dost světlý" and that is not the same.
Why is the answer marked wrong if 'it' is left out of the second phrase,
as a UK English speaker it seems redundant here.
What exactly did you answer? Please use the reporting feature "My answer should...".
"That house had few windows, and so was not very bright." is accepted.