When to use it
Use it when you genuinely can't take something on right now. A short, clear no plus the earliest time you could help is more useful than a long apology. The other person can decide whether to wait or find someone else.
Example
Colleague: “Can you review my proposal today?”
You: “I can't take this on this week. I could start Monday.”
Colleague: “Monday works.”
In an email or Slack message
In Slack: “I can't get to this week, but I could start Monday. If that's too late, Jordan might have time.” Suggesting someone else is helpful when you know they have capacity.