If the Calendar complication isn’t available, it is likely due to one of the following reasons:
The Calendar complication isn’t supported in that location on the Watch Face. Open the Sugarmate app, go to the Setup page in the Apple Watch section, then make sure you choose a supported watch face in Step 2 and a supported location for the complication in Step 3.
You don’t have Apple’s Calendar app installed. To install it, open up the App Store on your iOS device and search for Calendar. Then download the Calendar app (white icon with the date in black and red) and try again.