Some of my users are unable to see this tab under the activities timeline. I tried a solution where I would change the record type to a custom record type, but that makes it disappear from my view (System Admin profile).
If we have any custom record type on the Activities(Event or Task), we need to create a global action and define the record type on it and add it to the layout.
In my developer org, I have created a record type for Event object and so create Event button disappeared from the activities. After I created a global action and specified the record type and added it to the layout, it again appeared.
See related posts :