Depending on the sync mode Allocatus can sync MS Project tasks as events or appointments. An exception are absence tasks. For absence task you can link existing appointment and events from your calendar with task in MS Project. Since absence time can be only for a couple of hours (appointment), an all day (event) or for a couple of days (multi-day all day event) Allocatus supports also a mixture between appointments and events for a single task.
How ever for one MS Project task and a single day you can only have an appointment or an event, since you are absent either for some hours for the entire day. The error message means that you have linked at least one appointment and one event for the same task for the same day. To avoid this, please either assign the conflicting appointment or event to another task or delete it.
Note, that 16 hours are the maximum duration for an appointment. Multi-day appointments are not supported, use a multi-day all day event instead. Also you cannot have two or more appointments or events linked to the same task at the same time.