Hi,
Appreciate you reaching out and explaining the situation. That sounds incredibly frustrating, especially after just purchasing Microsoft 365 and expecting it to work right away.
When it says your purchase was successful but can't associate your account, it usually means there's a mismatch between the account you used to buy the subscription and the one you're trying to sign in with in the Office apps. Sometimes, Microsoft 365 gets confused if you're signed into multiple Microsoft accounts on your Mac or browser.
Here’s what I suggest to get things sorted, First, double-check which Microsoft account you used to make the purchase. You can do this by going to https://account.microsoft.com/services and signing in with the account you think you used. If the subscription shows up there, that’s the right one.
Sign out of all Microsoft accounts in your Office apps and browser, then sign back in using only the account that shows the active subscription.
If you're still getting the error, try opening Word or Excel, go to the top menu, click Help > Sign Out, then restart the app and sign in again.
Hope this clears things up and gets you back on track. Let me know how it goes or if you run into anything else.
Best,
Edward