Recurring Events that are created in Lightning Experience behave differently from recurring Events created in Classic.
To delete the whole series of recurring Events created in Classic, we need to open the Event series and then click the 'Delete Series' button. If a recurring Event is created in Lightning, the 'Delete Series' button is not available.
Note: These steps only apply to an Event series created in the Lighting Experience.
Delete button not available on Events and Event series from Lightning Sync
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