In addition to the options Asia said, it looks like you were trying to say: "we could have gone there for lunch today, if you hadn't promised to go with someone else". Because the "if" portion of the conditional is in the past tense (the antecedent), you need to put the consequent clause in the past conditional as well ("we could have gone", instead of "we could go"). Otherwise, you're mixing verb tenses.

The sentence is a bit confusing, but a good start. you can say: We can go there for lunch, if you're not busy. I want to go there for lunch, would you like to come? If you're free, we can go to lunch./go there for lunch. If you're not busy, would you like to go to lunch? If you don't have any plans for lunch, you can join me.

Better maybe to say “I think we could have gone there...” it doesn’t sound right this way you have it.