This most commonly happens if you previously had the game installed on a drive and have since removed that drive. To resolve this, take the steps below if you are comfortable editing your registry settings.
Note: Any deviation from these steps to edit your registry settings could cause significant problems for your system, so use caution when making these changes.
- If any version of MTG Arena appears in Control Panel → Programs and Features, uninstall it
- Open regedit.exe
- Navigate to HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node
- Delete the Wizards of the Coast folder
- Redownload and install the game from https://mtgarena.downloads.wizards.com/Live/Windows64/MTGAInstaller.exe