Unable to change or add Individual Sharing Policy

Sharing in Exchange Online is not enabled by default, so you shouldn't need to change anything. And creating a new policy will not affect the default configuration, so if you are trying to prevent users from sharing externally, do that instead. Under Individual sharing select the Default sharing policy > Manage domains > select the Sharing with everyone entry if present and remove it. If you also want to disable Calendar publishing, remove the Anonymous entry as well. Alternatively, instead of deleting you can Edit the entry and toggle the Share your calendar folder checkbox.

If for some reason the process fails in the UI, use PowerShell as detailed here: https://learn.microsoft.com/en-us/exchange/sharing/sharing-policies/modify-a-sharing-policy