Looks like whoever you bought your laptop from installed a pirated or non registered legit copy. Short of buying a real license, there is no legal way to get rid of the banner that I am aware of.
I believe it’s common practice for Windows laptops to come preinstalled with Office 365 now. I don’t think it’s pirated. It’s just not licensed like you said.
OP, I’d recommend just using LibreOffice. It works fine for most situations.
The Office that comes pre-installed is often just a trial, and you need to buy a license or a Microsoft 365 subscription to use it. LibreOffice is a good free alternative, if you don't specifically need the Microsoft applications, and it will read and write MS Office files.
Unless you specifically paid extra for a license of ms office, this is just the trial crap bloatware they include with new PCs. You still have to pay for it if you want to use. I’ve heard you can get really cheap activation keys from certain “cd key” sites.
If you go to the "about" section, it will give you more detail. Or, if you brought it from a reputable shop, bring it back to them and ask them why it says this.