Learn English – “I wish I could” or “I wish I could have”

This really is a matter of idiom—what people actually do say—not grammatical or logical meaning.

In strict logic, all of these amount to the same thing, that it is possible that they are right now at their expected destination:

• They can have arrived (by) now.
• They could have arrived (by) now.
• They may have arrived (by) now.
• They might have arrived (by) now.

Could or might suggest somewhat more uncertainty than can or may. Can and could address their command of the physical ability or resources, while may and might address the probability.

But in fact, the past form in this case will usually point to an implied unreal or highly unlikely contingency; and people are unlikely to use can or could in these circumstances except to draw a contrast between possibility and expectation:

• They might have arrived now—if they had left on time.
• They could have arrived now—if they had had a strong tailwind.
• They can have arrived now—but I doubt they have.

So what you probably want is

• They may have arrived now—why don’t you call and see if they’ve landed?

I wish you could have come to the party.

I'm sorry you could not come to (attend*) the party.

Both acknowledge that (for whatever reason) you wish they could come and you are sorry they missed it.

(*attend means 'come to' or 'be present at')