If you only have a few days left then it probably is effectively "no" if your professor is super busy. If you don't hear back on something important like this, you should forward the email again after about a week, politely asking for an acknowledgement.
If you are lucky enough to study somewhere where your professor has an administrator, you should contact the administrator rather than email the second time.
It is also possible that your professor has instructions on their Webpage about how to ask for letters of reference; I've seen that occasionally. If so, they may not think they need to reply to emails that ignore those instructions. But most people know that not everyone will read every Webpage.
You could forwarding the email again with "urgent" in the title, but if you do, write a short note at the beginning apologising for not having checked on them earlier and saying you realise this is probably too short of notice now if they missed your previous email, but you just wanted to double check.
Be sure you have sent them everything they would need to know for their letter, e.g. complete details of how they know you, where you are applying, how to submit the letter, etc.