Hello, how actually insert_joined_row action works?

Im trying to insert joined row from A to B, but getting 'Cannot access 'joinTable' before initialization' error message.

Here's my field to join :

A table with reg_number field (string, unique, required), its unique generated by fill_formula_button at view.

B table with reg_number field key to A.reg_number.

at configuration it has relation A.reg_number but getting that error message.