Hello I cannot complete the dewford city quest so I cannot unlock the second gym.I first went to the man who was standing in front of the gym and he told me to go to the cave I entered there and went to Steven and he said that he was busy then I went to Mr.Stone but he is giving me no letters please tell me is it a bug or it there any problem in my steps?

Speak to the man (devon employee) outside of Rustboro gym. He will ask you to go to rusturf tunnel and get what was stolen off him by the magma/aqua grunt. If you've done that speak to him again and he will tell you that he told Mr. Stone about you and that Mr. Stone wants to meet you. You will now be able to get the letter off Mr. Stone.