What causes the 'can't set tags: user not found' error during an 'Update User Profile' step in Customer Journeys?

When an Update user profile step cannot apply Tags to a traveller, the journey drops that user and names the reason in the point's Drop-off block. Two different reasons look similar, and the older internal wording "can't set tags: user not found" maps to both:

• User not found — the UserID that entered the Journey is not (or no longer) a registered user: the profile was never created, or it was deleted. Typical when events are posted for users that were never registered, or when an email address is used as a UserID without being associated with the user profile. Check your registration flow and make sure UserID values are not deleted.
• User mismatch — the device's current UserID no longer matches the UserID that entered the Journey. This happens when the UserID linked to a device changes mid-journey (log out / log in as somebody else, or a UserID reassignment in your integration). Journeys track travellers by UserID, so the step can no longer resolve the original profile.
• Tag not found — the Tag referenced by the element was deleted after the Journey was activated. Recreate the Tag or point the element at an existing one.

To reduce these drop-offs, keep UserID assignment consistent for the whole user lifecycle (especially around login/logout), register users before they can enter a journey, and avoid deleting Tags used by active journeys. Expand the Drop-off block on the element to see which of the reasons above you are actually hitting.