I maybe tagged for not being too detailed in this post but, how do you create a calendar in SharePoint 365 or 2019? I looked in Site Contents (or whatever it is on 365/2019) but I don't see how?
Select Settings , and then select Site contents.
Select + New, and then select App.
Select 'Calendar' app from the list of apps.
- Type the desired name you for your calendar app and click on Create.
More details can be found in this Microsoft Article.