Well, this is a very common issue among merchants, when they are using UpTrack for the very first time. There are two solutions to this issue.
Solution - 1
Sometimes, while trying to save your PayPal account client ID for the first time, you'll first need to refresh the server cache. So, just click the "Revoke Access" and re-enter the details which should be saved successfully.
Solution - 2
You might have created the PayPal Client ID and Secret already but they do not get saved and returns an exact error of "Client Authentication failed, Client ID or Secret is invalid."
This mostly happens when you unknowingly created a PayPal "Sandbox" App, rather than creating a PayPal "LIVE" App.
In order to create a Live App, kindly click on "Live" as shown in the below screenshot with the arrow marked and then follow the video tutorial below it.