Hi Vishal,
Actually, for an input field the node should be with cardinality either 1...1 / 1...n. i.e. at least one element should exist. So you need to pass "is_mandatory = abap_true" while creating node.
Please modify your code as below
CALL METHOD lr_root_node_info->add_new_child_node
EXPORTING
name = 'INPUT'
is_mandatory = abap_true
is_multiple = abap_false
is_multiple_selection = abap_false
* is_initialize_lead_selection = abap_false
RECEIVING
child_node_info = lr_input_node_info.
Hope this helps you.
Regards,
Rama