I can not be online

You need a gold membership to play online and your account's age should be 18+

Check the status of your gold membership or if you have been suspended. Do you get any error code when you try to play online?

keep us posted

Yes, you can redeem codes online. Here's the link:
account.xbox.com/.../RedeemCode

Make sure you are signed into the correct account before redeeming though as content cannot be transferred between accounts.

Once redeemed, the game should begin to download automatically on your console. If not, go to My Games & Apps and scroll all the way to the right to the section called "Ready to Install" and you should see a tile for Halo 5: Guardians; simply hit A to begin
the download.

Let us know how you get on.

This seems to be a common issue with people who added gold subscription over the free gold weekend. You'll need to try contacting support. 1-800-4my-xbox if you're in the US/Canada or
support.xbox.com/contact-us

@NJDevils - He redeemed it on October 4, not April.

I would suggest trying a power cycle. You can perform this by holding the power button for 10 seconds until the console turns off and then unplugging the power cord for 10 seconds. Once that is done plug it in and fire it up, hopefully that solves the problem.