Hello Lykyan,
Thank you for using Apple Support Communities. Let's check your settings to make sure everything is set up correctly. Follow these steps:
On the iPhone:
1.) Open Watch app.
2.) Tap on Calendar.
In this section under "Calendars", which option is selected here? Do you have "Mirror my iPhone" or "Custom" selected?
If you have "Custom" selected, tap on it. If you see that iCloud is the only calendar selected, then go ahead and select the other calendars you want to appear on your Apple Watch. If "Mirror my iPhone" was selected, then follow these steps:
On iPhone:
1.) Open Calendars app.
2.) Tap on "Calendars" at the bottom of the screen.
From here, you should be able to select the calendars you want to mirror on your Apple Watch. We hope these steps are useful. Thank you for using Apple Support Communities.