Virtual fields in table (child doctype)

how can I set the virtual field in a table (child doctype) and fill it by python code in “option” section in custom field settings?
In classic doctype I have no problem, here I set the field as virtual and put some “code” to “option” section and heureka :wink:
But in child doctype the same process is not working, unfortunately.

I have light table “Project Contacts” and for testing I need fill the “virtualfield” with now_datetime() value:

The “virtualfield” settings is here:

What am I doing wrong, please?