The user is not able to select a specific user to complete a workflow User Task in Vault.
The affected user is the Workflow Owner.
If the User Task in question is configured so that the Workflow Owner is not allowed to be assigned the task, the affected user is not available to be selected to receive this User Task. The solution is to either assign the User Task to a different user or cancel the workflow and restart the workflow as a different user.
Vault Document Workflow Documentation: Configuring Document Workflows: Workflow Task & Decision Steps