Default Editor – columns not printing [closed]

The columns appear vertically because when printing, the viewport is considered quite narrow. For the column blocks to appear horizontally next to each other, they have this in the CSS for the core/columns block:

@media (min-width:782px) {
  .wp-block-columns {
    flex-wrap:nowrap!important
  }
}

This media query does not pass when printing, thus the columns appear vertically. You may wish to add your own CSS to apply this when printing, for example:

@media print {
  .wp-block-columns {
    flex-wrap:nowrap!important
  }
}

You may also wish to qualify this with an extra class to ensure the above would not affect any other columns blocks:

<!-- wp:columns {"className":"foo",…} -->
<div class="wp-block-columns foo">

@media print {
  .wp-block-columns.foo {
    flex-wrap:nowrap!important
  }
}