In requests, both are correct and have the same meaning. Could is used for more formal or polite situations.

In talking about abilities and skills, could indicates past tense:

• I can play the piano.
• I could play the piano when I was only three.

When talking about availability in the negative, we can also use could.

• I’m sorry that I couldn’t come to your party last night.

However, in the positive, we use ‘able to’ for this purpose:

• I finished my homework early, so I was able to go to the party.