We can use 'could have' to talk about something somebody was capable of doing but didn't do.
I could have gone to Oxford University but I preferred Harvard.
Often, there is a sense of criticism.
You could have phoned me to let me know.We can use 'couldn't have' to talk about something we were not capable of doing.
I couldn't have managed without you.
We can use 'could have' to speculate about what has happened. (We can also use 'may have' or 'might have' in these situations.)
She could have taken the earlier train.
See this link:
Learning English | BBC World Service
Student or Learner