Once signed into eRin - https://erin.yodel.co.uk/ paste the tracking number into the search bar and click search.
You can track any order from this page, see https://beer-hawk-cs.notion.site/How-to-Track-an-Order-90201f3e6e164a2b9a070ebecdcb7416 for how to track an order
(I would always advise letting them know the customer has checked everywhere and with anyone that could have taken it in for them if it is a missing parcel, otherwise yodel will often just provide the details you can see on the tracking and say ask the customer to check around the property and with neighbours. The customer has probably already do this as we suggest it in our canned response for missing parcels)
You then just need to wait for Yodel to reply, sometimes they come back very quickly but it can take up to 48 hours. You need to check if you have any Yodel replies regularly.