Apparently "It's" is a typo and should be "It is" according to Duolingo. 奇怪的绿鸟!

'Not serious' was marked wrong. Why do we have to add the word 'it's ', when it does not say 这个不严重

Don't be worried, it's not serious. 这样行吗

I put "It isn't serious." and Duolingo said I had a typo! ("is not" = "isn't")