A This Patient transfer function is only available for patients where a customized transfer function was created. The customized transfer function can be promoted and turned into a custom template transfer function. By doing so, it becomes available for all patients.
To do so:
Open the Transfer Functions gallery.
In the This Patient transfer functions section, select the transfer function you want to turn into a custom transfer function.
Click Make Custom Template at the bottom of the transfer function gallery.
The selected transfer function is moved to the Custom Templates transfer function section.