The most likely reason is that your Resident Visitor Permit (RVP) has expired.
All visitor permits are valid for between one (1) and three (3) years from the date of your application being accepted. Therefore, once you exceed the relevant expiry date (regardless of whether you have used any entitled threshold) the RingGo system will recognise that your permit has expired and you will need to re-apply either with RingGo or the Council (Richmond residents only) in the same method as before. This is primarily to prove to the local authority that you are a valid resident and entitled to the RVP scheme.
Another reason may be that the telephone number you are calling from is not linked to your RingGo permit. Your registered mobile or landline number(s) must be used when calling our Permit Booking Line on 0843 209 0088 in order to validly purchase a permit session. Only the stored number can be used to purchase permit sessions by phone unless you hold a London Borough of Richmond Council RVP where you can store up to five (5) telephone numbers to purchase visitor sessions.