Can Microsoft activate an Xbox code that says "not yet active"?

No. Microsoft Support can verify whether a code has been activated and confirm its status, but they cannot activate it themselves. Only the retailer who sold the code can activate it. If your seller refuses to help or is unresponsive, your best option is to dispute the charge with your bank or PayPal — an inactive code qualifies as "item not as described." Always keep your order confirmation email until your code has been successfully redeemed.