This error means that the central NHS databases do not show a registered GP practice on your record. This suggests you have either been deducted (removed) from your GP practice's patient list or your registration with your GP practice has not been completed fully. The NHS App works by displaying your medical record from your GP practice and without a registered GP practice on file the NHS App will not work.
To resolve this matter please contact your GP practice. The Administration team or Practice Manager are best placed to investigate this issue on the Health Authority/GP Links system. They will be able to either re-register you or explain what you need to do next.
Should you require access to your COVID Pass in the meantime, please use https://www.nhs.uk/conditions/coronavirus-covid-19/covid-pass/ to see the options available to you.