Because you're either using an incorrect auth link, clientId or redirect_uri.
Solution
An incorrect auth link
Make sure that the Authentication Link you're using is valid for both live and sandbox environments.
Live: https://auth.truelayer.com
Sandbox: https://auth.truelayer-sandbox.com
An incorrect clientId
Make sure that the clientId that you're using is valid for both live and sandbox environments.
Example Live: truelayersupport-1a2b3c
Example Sandbox: sandbox-truelayersupport-1a2b3c
You can check this in the TrueLayer Console, under App Settings.
An incorrect redirect_uri
Make sure that the redirect_uri that you're using is valid.
Example Live: https://console.truelayer.com/redirect-page
Example Sandbox: https://console.truelayer-sandbox.com/redirect-page
Check the URI that you are using is whitelisted in the Console under App Settings.
More
If you need more information...
Have a look at our comprehensive list of the authentication error descriptions in our .