That was my answer too.
“我生病了， 所以不能上课" Still marked wrong as of 2019-04-08. Reported.
Is the 'le' required here?
How come you have to use 我 in both clauses?
I also wrote it without the second 我. Is it necessary here?
Why is ke yi needed?
The 所以 in the answer isn't keyi, but suoyi which means "therefore", "so" or "as a result".