To sign off a review task, you need to provide the Signoff object as supporting object rather than the user.
You need to create the Signoff entity like so:
This allows you to get the list of signoffs for the EPMTask
Then you need to go through the list of signoffs and find the assignee which matches the current user - this is your signoff
Change the signoff to give a decision:
Then the PerfomActionInput looks like this:
Can you provide a flow for completing review task.