Devon Goods Quest Bug

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After talking to Mr.Stone (head of Devon Corporation) he said he gave me a letter to deliver to Steven in the cave in Dewfort town. I arrived there and after chatting with Steven he did not leave and when I checked my bag I had no letter (I do not know if Mr.Stone gave me one to begin with). When i revisited Mr.Stone he says thanks for delivering the letter and gives me devon's goods to deliver to Devon in slateport (I have the item in my bag).

 

My problem is that to go to slateport I have to beat the 2nd gym but the entrance is blocked by someone who won't move until i get steven to leave the cave. Because I cann't get steven to leave the cave because I have no letter I am currently stuck in the game. Can someone help me with this problem?

 

PS. Let me know if I need to provide anything else and also I have watched some PRO tuitorials such as this one:

https://pokemonrevolution.net/forum/viewtopic.php?t=31625