There are several types of calendars in the Jira calendar, and depending on these types, the permission settings are slightly different.
Now I will tell you everything:

Basic Calendar - Such a calendar does not use issues from Jira, so we need to make sure:

• Make sure the user has access to the application itself. You can do this here:
  System-> Global Permissions-> Calendar Users

  The user you want to grant access to must be in this group.
  Most likely, all your users are in this group by default.

• Give the user one of the two calendar roles. Admin or User.
  The admin can change the calendar, as well as add new events on the calendar. The user can only view the calendar.

Project Calendars (Project, JQL, or Filters)

• Global permissions
  Make sure the user has access to the application itself. You can do this here:

The user you want to grant access to must be in this group.
Most likely, all your users are in this group by default.

• Filter and Issues permissions
  Only for Filter Calendar - Make sure that the user has access to your filter and has access to issues in this filter. https://support.atlassian.com/jira-cloud-administration/docs/manage-shared-filters/
  Project calendar, make sure the user has access to the project the calendar is built on
  JQL calendar, make sure the user has access to the project/projects the calendar is built on

• Calendar permissions
  Give the user one of the two calendar roles: Admin or User.
  The admin can change the calendar. However, the user can only view the calendar.