My flush tank system from leakage?

Leakage from wall concealed toilet tank push button panel . Leakage is basically from connecting tube inside, on outlet end. Local plumber could not repair; he feels one washer on output end is missing & this size of ( small) washer is totally missing. This washer is at the end and rests against the wall of nut . I don’t know if washer can be inserted or connecting tube is to be replaced.
Pl Help as I am not able to use the toilet flush. This is new system; although installed few months back but never used as cottage was unoccupied

D K Bhagotra
May 2021