Specific Robots tab not showing up when create "Start a Job"

Hi everyone,
I’m using Enterprise trial edition 2021.4 version. When I am trying to start a job and want to choose specifics robot, but specifics robot radio button is not available, just show “Allocate Dynamically” tab. Do anyone let me know? And how I can run more than 1 robot at the same time.
Please help and advise on how to resolve.