AWS was not able to validate the provided access credentials

Hello.

Try running the AWS CLI command with the "--debug" option added.
You may be able to determine what the problem is by looking at the debug logs.

aws ec2 describe-instances --debug

I also think you need to make sure you are using the correct credentials as mentioned in the documentation below.
https://repost.aws/knowledge-center/troubleshoot-aws-cli-commands-ec2

aws sts get-caller-identity