May 31, 2020, 04:5605/31/20
Can no longer upgrade rank 3 adventurers to rank 4

It's probably a bug because you certainly did it before and the rules didn't change.
To look at it we need your identity in the game, in case you don't know where to look at it I include the image.
Seguramente se trate de un bug pues ciertamente anteriormente lo hiciste y las normas no cambiaron.
Para mirarlo necesitamos tu identidad en el juego, por si no conoces donde mirarlo incluyo la imagen.