Read the error in detail:
It’s telling you that you can’t have a reverse proxy upstream to a subfolder. You may only specify a scheme (e.g. https://), a host (e.g. example.com), and a port (e.g. :8080). Trying to proxy to a subfolder implies a rewrite, which reverse_proxy doesn’t handle.
Instead of defining an upstream with a URI, manipulate the URI yourself before sending it upstream, e.g.
# Prepend '/dps'
rewrite /dps{uri}
# Send upstream
reverse_proxy https://upstream