Both sentences can be correct 🙂 “He called to say he can’t come.” → This is used when the situation is still true now (he still can’t come). “He called to say he couldn’t come.” → This is more common in reported speech, especially when you are telling the story later. In everyday English, “He called to say he couldn’t come” sounds more natural when you are reporting what he said. So grammatically both work, but the second one is usually preferred. 👍