For anyone out there that is finding it hard to use "Attack 1" via clicking LMB, just make sure that you don't have a controller plugged in. If you don't and you still have that issue, try going into your settings and unchecking the "Gamepad Enabled" box. Just posting this here, because I didn't see it anywhere on the Steam page, and figured it wouldn't hurt to do so for those having the same issue. But yeah. No clue why it does that, but it does and that solution works. Hope this was helpful to at least one person.