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?
I've never seen 生病 used without 了.
How come you have to use 我 in both clauses?
Would it be OK to say 不能参加课？
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".
Still how important is it here? We get the same meaning without it...
所以 in this case is like saying 'therefore' as LeiFeiRalf says.
So it makes being sick the reason or cause as to why the person can't attend class.
"I am sick, therefore I can't attend class".
That 's different to "I am sick, I can't attend class".