How can I add a so i can show sale price under normal price

Why not just use CSS rather than altering the template for such a minor markup change that’s really only serving a styling purpose anyway.

.electro-price > ins{
   display: block;
   float: none; /* <-- In case the theme styles have it floated */
}

You may need to use slightly different styling depending on what’s already being applied to the element, potentially even the !important flag to force the override, but that’s much better than handling a style issue with a template change or JS injection.