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, depending on the operator or council. Therefore, once you exceed the relevant expiry date (regardless of whether you have used any entitled threshold, if your operator or council uses thresholds) the RingGo system will recognise that your permit has expired and you will need to re-apply either with RingGo or the Council (London Borough of 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 020 3046 0005 in order to validly purchase a permit session. Only a stored number can be used to purchase permit sessions by phone. Some operators or councils allow you to have multiple telephone numbers on your RingGo account.