AWS was not able to validate the provided access credentials

Using the latest version of the Postman app on MacOSX 10.14.3

I’m trying to do a simple DescribeInstances AWS API call but I seem to be
doing something wrong as I get a 401 and “AWS was not able to validate the provided access credentials”.
My Mac is set to set the time automatically and I can do any AWS CLI call like
“aws s3 ls” with no problem. I’ve taken the access key and secret key that
I’m using in Postman and specifying them on the command line for an
AWS CLI call to make sure that they work and they do.

Of course I have Auth set to “AWS Signature” and Service Name to “execute-api”.

Any suggestions?