"I BOUGHT A MICROSOFT 365 SUBSCRIPTION ..."
Actually, no. That's not how it works, technically. Your Microsoft account bought and paid for a subscription, and it's that Microsoft account which owns the subscription. You own your Microsoft account.
So, after you have installed the apps to your computer, you have to sign in to the apps with the username and password of the Microsoft account that purchased the subscription. This is easy:
Launch any of the Microsoft 365 apps - let's say Word. Go to File > Account > User information and enter the username and password of the Microsoft account that purchased the subscription. That's the only username and password that will work. (If you see a different username and password already there, click on Switch Account.)
Once you've done this, your subscription is activated to your Microsoft account. And since you own that Microsoft account, you can now use your software. Enjoy.