Thanks so much for your help @cintiadr. I’m stumbling my way through CI 101. I’m not sure how the Deployment Reference Application Deployment Project UAT 2.4 was created, but I used your advice to add the “bamboo” username to inline scripts and convert the SCP Task to an inline script and got the build to succeed. 
I took an extra step of moving the target username & server into variables set at the deployment project level. Not sure what conventions have been used for this (so, might be breaking them), but it seemed like a better approach than hardcoding user & server names in each step.
Thanks again!
-Burke 