In '_custom-styles.scss' I want to extend some bootstrap classes like py-2, for example

.myclass {
  @extend .py-2;
  color:green;
}

but I get this sass error:

Error in plugin "sass"
Message:
    scss/_custom-styles.scss
Error: ".myclass" failed to @extend ".py-2".
       The selector ".py-2" was not found.
       Use "@extend .py-2 !optional" if the extend should be able to fail.
        on line 13 of scss/_custom-styles.scss
>>       @extend .py-2;

   --------------^

How can I solve this issue?