M635 Owner not able to share calendar with internal user with previous access.

The owner of the M365 account can no longer share her calendar with an internal user of the account. User had availability to View the calendar before, but no longer has access. When owner tries to Share the calendar the following message comes up "You don't have permission to share your calendar with mailto:user@organization.com". User is part of the organization and I've confirmed is an Active User. The only difference I can find from other users is this user is using the the Apple version of Office 365 on an MacBook Air. Any direction on this would be helpful.