It can always happen that you get sick right before the gig or you are unable to come for some other reason. Initially, as a freelancer, you are required to arrange a replacement. Always do this in agreement with the client because otherwise they may simply reject the replacement. 

Can't find a replacement, and you really can't go to the gig yourself? Cancel the gig via the app. This way, the client knows you will not come to the gig. If you cancel the gig after the cancellation period ends, you shall still receive the gig not completed notification.

When you receive a 'gig not completed' notification, it affects your attendance rate. The more gigs you don't complete, the lower your attendance percentage becomes. Clients see your attendance percentage when you respond to gigs, so make sure you keep your percentage as high as possible.

Read more about the cancellation policy here.