Why can't I delete an invoice template?

There are two reasons Delete is greyed out on Settings → Store Settings → Invoice Templates:

• Built-in templates (Classic, Compact / Receipt and Modern) can never be deleted. You can still Edit them.
• The Active template can't be deleted, because Print invoice uses it. Click Set active on another template first, then delete this one.

Hover over the greyed-out Delete to see which reason applies. Deleting a Custom template asks you to confirm and can't be undone.