Callings - how to mark "set apart"

I can add a calling for a member, and I can edit that calling. However, I whenever they are set apart and I go in to tick the check mark box for that and try to save it it says the calling already exists. And if I exit without saving it doesn't save the checked box. How do I mark calling as set apart?

Go to the Organisation’s tab instead, to “Set Apart”.

This is a long running and reported bug. Maybe it will get fixed some time. Fortunately there is the workaround already described above.