Can you please provide a video clip or screenshots of what you're experiencing? If not, please double-check that you have Beyond Light downloaded and installed on your PC. If you were logged into the game when making the purchase, quit and check if the DLC is waiting to be installed or if there are any other updates available. After confirming the DLC is installed, log into the game, pull up the Destinations screen and travel to Variks in Charon's Crossing. He will provide the campaign quest. [quote]Note that there are 2 separate versions of Beyond Light available from the Microsoft store: 1 for Xbox consoles, 1 for Windows PC. If you accidentally purchased the version for Xbox, then you bought for the wrong platform and would need to contact Microsoft for any refund option so you can purchase the correct version or to see if they can do an exchange.[/quote]

Navigate to the same store page where you made the purchase to install, or from the Xbox games app on your PC.

I suggest you contact Xbox Support to figure out what you actually purchased and what your options are. It sounds like you bought for Xbox, not PC, which would explain why you're seeing "Buy" instead of an option to install. They can also guide you through installation.