AWS CLI - Error - AWS was not able to validate credentials

I have set up AWS EC2 instance through AWS Management Console and able to launch it.  While using AWS CLI to connect to instances I encountered the error "AWS was not able to validate credentials" error.

I have carried out below steps:

• set up AWS instance through Console
• Create Access Keys and downloaded the rootkeys.csv file
•  On Linux machine, installed Python 3.6
• Installed AWS CLI tool
• Configured AWS CLI on linux machine using  the command - aws configure

After all these steps, when I run the command - aws ec2 describe-instances,  I was encountering the error "AWS was unable to validate credentials".

I verified the AWS credentials file, all the details like AWS Access ID and AWS Secret Key are all present.

To resolve this error, I had to explicitly export the variables AWS_ACCESS_KEY and AWS_SECRET_KEY.

After this I am able to connect to the running instances from AWS CLI.