Hello Lilyana J,
Welcome to Microsoft Community.
I am a Microsoft user just like you, I hope you are doing well.
If you have Microsoft 365 product key, you need to redeem the product key to your Microsoft account in order to install and activate Office on your PC. The redemption process shouldn't require a payment method but if you are prompted to, please specify at which point you are prompted to enter a payment method.
To redeem the Office 365 product key:
Step 1: Go to www.office.com/setup or Microsoft365.com/setup.
Step 2: Sign in with your Microsoft account, or create one if you don't have one.
Step 3: Enter your product key, without hyphens, if prompted.
Step 4: Follow the prompts to finish the redemption process.
Once you have redeemed the product key to your Microsoft account, you can then install Office 365 by signing in https://office.com/myaccount on the PC.
You will find Office 365, click install. Once you have successfully completed installation, you need to sign in your Microsoft account in any Office app to activate Office.
Find guidance to install Office: https://support.microsoft.com/office/4414eaaf-0478-48be-9c42-23adc4716658
Source: https://support.microsoft.com/office/0a82e5ae-739e-4b92-a6f4-2ec780c185db
Please let me know if you need more assistance.
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Warm Regards