Windows 10 version 2004 "Your Device is not ready"

I have the same message before, but then it changed to "The Windows 10 2020 Update is on its way. Once it's ready for your device, you'll see the update available on this page". So now it is clear that the update is not yet compatible on my device, instead of telling me that my device is not yet ready as if I have to do something for it to get the latest windows update.

No. That is incorrect.

Without the quote, I had to search back to see who you were and what you meant.  If you are saying that you don't see your product key on the screen after Start -> Settings -> System -> About, you may be looking for the activation code.  On my computer, on the About screen there is a block of data under the heading Device Specifications.  Listed are device name, processor, installed RAM, device ID, product ID, system type, and pen/touch information.

If you are asking for the activation code, click on the link at the bottom of that screen "Change product key or upgrade your edition of Windows" to bring up the Activation screen.  On the top right there is a heading "Where's my product key?" and a link "Finding your product key."

Look on the back of your computer or the bottom, if it's a laptop or tablet, and if it was sold with Microsoft Windows installed there will be a sticker with the product key on it.  If Windows was installed later, the activation code will be on the box the media came in.  That code should activate Windows on that computer if it has a legitimate upgrade.  Most computers sold with Windows have the activation code in their BIOS and Windows activates automatically.

Upgrades often have a digital license.  That is embedded in the system and encrypted, and is associated with a Microsoft account and your login PIN/password.  If your Windows is not properly activated and you don't have an activation code, ask the person/outfit that sold you the computer.  If that's not an option (eBay seller, etc.) you need help from Microsoft.

No. That is incorrect.

Asking for product key.I bought this pc from someone

Misplaced question.  As another user, I will say that the product key can be found from the System Information screen: Settings -> System -> About.

The thing is I need to do a reinstall because I have audio issues and it seems that only a reinstall solves it. If applying the update solves it, that saves me a whole lot of work sorting out things after a reinstallation so I would rather go that way if possible. For now I am just watching youtube on the laptop beside the computer, no time for gaming so not a huge loss right now having audio breakups that noone seems to know why other than a fresh installation will not have them.