It is not possible to select the Visit option to add as reference to a task.
Reproducing the Issue
Go to Activities work center.
Go to the Appointments view.
Open Appointment ABC(ABC represents the Appointment ID).
Go to the Related Items Facet.
In the Related Items section click on Add.
In the drop down list for the field Type you are not able to find the Visit option.
This is the current system behaviour.
The Visit option is not available in the drop down list in the current release of SAP Cloud for Customer.
We have addressed this to Product Management and they have added this to the backlog for future release.
KBA , LOD-CRM-ACT , Activities , Problem