I faced with the same issue. I just added credentials config:
aws_access_key_id = your_aws_access_key_id
aws_secret_access_key = your_aws_secret_access_key
into "~/.aws/credentials" + restart terminal for default profile.
In the case of multi profiles --profile arg needs to be added:
aws s3 sync ./localDir s3://bucketName --profile=${PROFILE_NAME}
where PROFILE_NAME:
.bash_profile ( or .bashrc) -> export PROFILE_NAME="yourProfileName"
More info about how to config credentials and multi profiles can be found here