Flag error in insert preamble

I am trying to implement ofdm using grc blocks. I am getting an error in
insert preamble block saying

TypeError: in method ‘ofdm_insert_preamble’, argument 2 of type
'std::vector< std::vector< gr_complex,std::allocator< gr_complex >

,std::allocator< std::vector< gr_complex,std::allocator< gr_complex > > >
const &

I have attached the grc file. what should i give as input to flag input
of
insert preamble block and also how to connect the output flag of insert
preamble block. How does that flag work in the block.