Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
Which specific Office 2019 product is this? Office Home & Business 2019? Or some other version?
Note that you the product key can only be used once, during the initial setup. After that, you reinstall the product using your associated account at https://account.microsoft.com/services.
If you run into activation issues, try phone activation. Use the phone number for your region from the article at https://support.microsoft.com/en-us/office/-telephone-activation-is-no-longer-supported-for-your-product-error-when-activating-office-9b016cd2-0811-4cb3-b896-5a6a13177713 (even if you don't see the exact error message mentioned in the article).