Keep in mind that nowadays, not every Office installation needs a key. Older versions might, but subscription products or newer software may just associate itself with your Microsoft account and payment details, rather than use a traditional software key.
So, the first thing to try is logging in to your Microsoft account and going through the setup instructions at office.com.
If you’re positive you need a license key, get in touch with your IT department or Office administrator. If you’re an individual user, Microsoft support will be your best bet.