Cannot create thumbnail on post image upload

Hi, i’m trying to create the tumbnail after the image is uploaded, but the <select> that let’s me to change the size of it is disabled and i can just post in the original size. First the gd extension was not installed but after installed it nothing changed. Any other thing to try? I’ve tried to search into the code why it puts to disabled but i haven’t found anything..

Any clue?

Regards