i hit this using the CLI previously on a freshly installed os image. if i recall the root cause was that my default AWS region was not set in aws config. error message is a a little non-intuitive since it implied the problem was with pulumi rather than the aws environment that pulumi was inheriting.
Codifying things like AWS region as a part of stack config is recommended. You can certainly inherit values from the ambient environment, but if you have a teammate or CI system try to run the project, config will make thing more reproducible.