The local calendars are intended to be saved only locally on the device of creation. This is a special type of calendar which is not linked to any account and therefore is not synced.

If you want to transfer a local calendar into a synced one, you will have to export the local calendar and import the file into a synced calendar. This in the settings -> maintenance -> export/import calendars (please see article here for more detailed information).

Once the sync process is finished and you’ve made sure all your data is transferred to the servers of the sync partner, you can remove the unnecessary local calendar to avoid doubling of your entries. This in calendars & task lists -> calendar -> local -> pencil icon -> bin icon.