Why don't I see my G-Coins in my account that I purchased yesterday? It was charged to my...

Hi MO! I'm Rostom, an Independent Advisor. I will be glad to help you here.

In some cases, in-game or item purchases may take up to 48 hours before it will reflect into our accounts. I suggest that we wait. We could try to refresh your console as well by power cycling it to see if it's going to refresh everything.

Press and hold Xbox's power button for 10 sec. Unplug the power supply for 30 sec. Plug it back in and turn it on again.

I need a refund but I don't know what to do I've tried everything

More information regarding refunds can be found here:
https://beta.support.xbox.com/help/subscriptions-billing/manage-subscriptions/digital-goods-subscriptions-refund

I was writing with a Microsoft agent now, and he said he didn't see on either of my accounts that the game was purchased. So I sent him a transaction code from the bank and said he'd check it out and they'd get back to him. Well, hopefully they'll bring
the game back to my library.

It just shows me that it can be purchased. I don't know how that's possible. I have bought the game, checked to see if it was the right account, but I still don't see it in the "my games" section... Unfortunately all I have as a proof is a bank payment.
I didn´t gain any e-mail purchase confirmation or something like that.

Have you checked on the Microsoft Store page of the game to see if it says you can install or purchase?