First, ensure that you're signed in to the correct Apple ID on your iPhone. Sometimes, issues with subscriptions arise when there's a mismatch between the signed-in Apple ID and the one associated with the subscription.
If you've confirmed that you're using the correct Apple ID and the problem persists, try signing out of your Apple ID on your iPhone and then signing back in. This can sometimes refresh your account information and resolve subscription-related issues.
If the problem continues after signing back in, I recommend contacting Official Apple Support for further assistance. They can help verify your subscription status and investigate any discrepancies between your Apple One subscription and access to Apple Arcade.