Variant reporting works on a sample level, i.e. you can report multiple variants belonging to the same sample, but you can't report one variant from the sample A and another one from the sample B within the same report.
In the Variant Table page, you can either select one variant at a time just by clicking on it or you can select multiple variants at once (mass selection) by holding Ctrl and clicking on each variant you wish to include to your list.
and then click on the “Select for export” option.
On the left, click on the arrow icon and then either choose or :