Using classic Outlook for Windows in business environments
To find the main calendar, go into the inbox and create a new event - New Item > appointment. The calendar it is added to is your default for that account. It should not trigger the reminder error.
What are the names of the other calendars? If birthday or holiday calendar, you need to remove them in Outlook on the web.
If you right-click on the calendar folders, is Delete Calendar grayed out?
In the folder list - are all 3 calendars in the same data file?
To open the folder list: Ctrl+6 to open it if the buttons are on the bottom. If your Outlook navigation buttons are in a vertical bar on the left, click the bottom icon in the vertical bar and choose Folder. To switch back to the Mail pane, click the Mail button or use the keyboard shortcut Ctrl+1.