In order to create a Google Translate API Key you need a paid account. Through their terms and services, Google doesn’t allow for non-paid usage of their Translation API.

https://cloud.google.com/translate/

To create your application’s API key, Go to – https://console.cloud.google.com/

1. Create a new Project from the top bar.

2. Select newly created project from top bar and visit API & Services.

From Dashboard enable Google Translate API

Search for translate API and enable it

3. From the credentials interface create a new API Key.

4. Copy this API key and insert inside wp-admin >> Loco Translate >> Auto Translator Addon – Settings.