In case you are noticing "Invalid Credentials" as the error message while installing the app on Freshservice, it could be possible that your API key is either invalid or does not have sufficient permissions. We would suggest to use the Org Admin's API key for the integration to function smoothly.
If you believe the API credentials are correct, could you please to share the Freshservice domain and the API key you are using? We will have it validated at our end.