How can I sync Calendar with iCloud successfully?

Hi, I cannot add any event in the Calendar since the plus sign is greyed out & I cannot view any events.

The calendar is synchronised to iCloud; I switched the synchs on and off; I checked if there are any updates for the app, I restarted the phone. Nothing works!

Could you please help me? Thank you!!

[Re-Titled by Moderator]

Original Title: Add or view events in calendar