To work with travelpayouts flights API module you need to signup with travelpayouts and then check under your API account for API access at https://www.travelpayouts.com/programs/100/tools/api
Normally the access for API would not granted and you have to contact travelpayouts for API access they will review your account and grant you access based on their policy and business checkup. Once API granted you will need to copy the API Token and Marker then login to your website as a admin then navigate to modules travelpayouts flights API -> ( settings ) and paste your credentials as shown in below image.
After that check your website by searching for flights and once you review the result please send your website to travelpayouts for preview and approval of site. And make sure your meet travelpayouts API policy before you get started here is the link : https://support.travelpayouts.com/hc/en-us/articles/210995808-Requirements-for-Flight-Search-API-access