I$\text{'}$m also having trouble with an internal client who keeps changing their requirements. I think this is causing extra work and costing us time and resources. What should I do$?$ What is thebbest option $?$

Raise this with your manager, highlighting your recommended approach for dealing with the issueIf the work is requested by an internal glient, you should follow this request to the best of your ability, even if you feel it may change.

Point out to the internal client the approach you think is most effective and make them aware of the impact of frequent changes