Hello ! I recently bought a king's rock (15k) from the Slowpoke Well B1F NPC seller, then tried to evolve my Poliwhirl with a trade, but it doesn't. I tried it 3 times and then realized that my item hasn't the ' from King's Rock (see screen attached)

 

Could it be instead that you're trading it to someone with a full Pokemon Party? If you do that, it will not allow you to evolve it, as they need one or two empty spaces in their party to evolve it. My Kings Rock looks the same but allows me to evolve pokemon.

Hello Bhimoso,

Thank you so much for this reply, now it's fixed because I think I didn't have two empty spaces in my party :

 

Have a nice day !