Final answer:
Rob cannot back-promote a user story because the Dev1 environment is locked due to environment configurations like a code freeze, deployment in progress, or workflow restrictions.
Step-by-step explanation:
Rob has encountered an issue where he cannot back-promote a user story to the Dev1 environment because it is locked. This restriction likely exists due to environment configurations which are set to prevent changes to certain environments. Such a lock can be implemented for a variety of reasons, such as a code freeze during a testing phase, a deployment in progress, or it could be part of a workflow where certain environments are only modifiable by specific roles or during certain times to maintain integrity and stability. Rob should check the environment's policy or workflow restrictions and collaborate with the project administrators or the operations team to understand the exact cause and find a way forward.