Hi Christos Christodoulou,
If it is just proof that you are paying National Insurance contributions in the UK, that you require, you can apply for a statement of National Insurance contributions by completing form CA3916. This can be found on the gov.uk website -
Get a PDU1 certificate or a National Insurance contributions statement
With regards to cancelling the other certificate that you have requested, please call the National Insurance helpline and ask them to cancel your application.
The contact number is 0300 200 3500 (+44 191 203 7010 if calling from abroad). Lines are open Monday to Friday from 8am to 6pm.
Alternatively, you can attach a note to the CA3916 asking HMRC to cancel your application.
Thank you.