I am sorry if this has been asked before. If I have say an Ion Canno equiped on IG-88B and fire and miss with it can I fire with it again per his on card ability?

There's no restriction on which weapon you use for the first attack - the stipulation is that you must use a Cannon for the second.

So yes

The only time this ability would not trigger would be if you did not have any cannon weapons equipped

Note that it is restricted to "once per round" so you can't keep on shooting until you hit

I understood that part like "Gunner" just wanted to make sure I could use the same cannon for both shots if needed.

Yes you can use it for both shots