Why does my step count not appear in my calendar?

Are the steps missing only on the calendar, or are there no steps for the last days on the Steps screen either?

In the first case, double-check whether Steps are enable in the settings of the Calendar (see the icon with the drawers top right of the Calendar screen).

In the second case, I recommend to remove and add the device From the Garmin Connect App, then log a few minutes of steps, sync, and see whether it helped. It won't fix the past data, but should help from now on.