New Web Form error: value missing

Hey guys,
I created a new web form from a custom doctype. First attempt to test it worked but the values entered in the web form were not appearing in the doctype. Also the doctype result was empty, no field appears.

Second attempt gives the following error:

Error: Value missing for None: parent
Error: Value missing for None: parenttype

Anybody can help please ? Using v13 here.

It seems the problem is the doctype can’t be
Is Child Table

But I would like it to be as I want to have the doctype displayed as a table in another event doctype.