Why can't I reinstate completed tasks on my iPhone 13 Pro Max?

Probably you are referring to the Reminder App.

If a task is marked as completed in the iOS Reminders app, it should appear in the Completed section. However, if it is not found there, it likely means the task was deleted rather than marked as completed.

Here are some possibilities for why a completed task might not appear in the Completed section:

1. The task was manually deleted – If a user deletes a task instead of marking it as completed, it will not be found in the Completed section.
2. The list was deleted – If the entire Reminders list containing the task was deleted, all tasks within it (including completed ones) are removed.
3. A sync issue occurred – If iCloud syncing is enabled and there was a syncing issue, the task might not be saved properly.

In the Reminders app, scroll down and tap Recently Deleted. Tap the More button, then tap Select Reminders. Tap the reminders you want to recover. Tap Move, then tap the list where you want to add the recovered reminders.

Use Reminders on your iPhone, iPad or iPod touch