Hey @warriorprince26, welcome to Discussions! My apologies for not getting back to you sooner.
Sorry to hear you are unable to start the game in Ubisoft Connect due to the error "Unable to start game" (thanks for providing the screenshot).
If I understand you correctly the issue occurs only in two games: Assassin's Creed Valhalla and Assassin's Creed Odyssey and you can still launch other games from your Ubisoft Connect Library?
We recommend trying the following:
• Restart Ubisoft Connect.
• Verify your game files. (Note that during the file verification Ubisoft Connect might trigger an update. If this is the case, please cancel the update.)
• Press on the Update button once the verification is done.
If this does not help, please perform a clean installation of Ubisoft Connect.
If the issue persists after following the steps above, do not hesitate to contact us again!