After creating my first schematic, I deleted all of the corners and origin point and can not place any blocks using my mouse. I can break blocks, and pick blocks, but can not place. Wondering what keybinds I need to use because it is not specified anywhere.

Do you have the Easy Place mode enabled? That prevents placing blocks anywhere except what the schematic indicates should be there. This includes preventing any item use (right click) anywhere else. So you should always toggle off the Easy Place mode after you are done placing blocks, or if you need to eat or do something else that needs right clicking.

Okay so that fixed it, I didn't even realise that I had that option set as true.

You can close your issues yourself, if it was a problem on your side.

I probably need to re-work the Easy Place mode, as I can see this becoming a really common issue for people, as it's not at all obvious that this is a side effect of it trying to prevent wrongly placed blocks. I can probably at least allow right clicks outside the schematic areas.

Also, maybe it would help do give some warning if player makes, for instance, 100 RMB clicks in a row, that do nothing, because of easy placement?

Yep I have also considered adding some kind of generic system to track unsuccessful or blocked input (mouse & keyboard) actions, and try to give an info popup about what might be the causes. But making that system generic enough for it to be easily usable for most things might be somewhat tricky.