Another trouble with Use CSS Animation to Change the Hover State of a Button

HTML-CSS
1

The test is passed successfully, but the preview does not work. When I hover the button, it does not change the color.
May be I did something wrong?

@keyframes background-color {
      100% {
        background-color: #4791d0;
      }
    }
2

Is this an exercise? If so can you paste link to it?

3

Sure.
https://learn.freecodecamp.org/responsive-web-design/applied-visual-design/use-css-animation-to-change-the-hover-state-of-a-button/

But hover animation works nicely in the next challenge.