Open the customer's subscription (Subscriptions > the subscription) and find the delivery day in its schedule. When that day is closed, still active (not skipped/cancelled), not yet finished, and still has unselected meals, an Auto Select Meals button appears next to it. Clicking it (after confirming) calls the same auto-select action limited to that one subscription and returns you to the subscription page, showing any error there instead of on the main dashboard. This is the quickest way to fix a single customer whose meals could not be selected in the bulk run after you adjusted their allergens, dislikes or plan categories.