Please provide Nexmo API credentials

I am working on an sms feature by the Nexmo package. However I encountered an error which is:

Please provide Nexmo API credentials. Possible combinations: api_key + api_secret, api_key + signature_secret, private_key + application_id, api_key + api_secret + private_key + application_id, api_key + signature_secret + private_key + application_id

Though I already filled up my .env and config/nexmo.php files.

via Chebli Mohamed