You must be on the Enterprise,Pro, or Pro Trial licensing plan to use this feature.
Robot unit reserve
For Cloud Robot - VM machines, once a machine is created and the RUs are consumed, we also make note of the required RU quantity and internally reserve it for the following month.
What this means is that when a month starts, the required RUs for your existing Cloud Robot - VM machines are automatically deducted from the tenant’s monthly RU pool to make sure they can run for the month.
For this reason, you could find that the robot unit amount for a tenant can be lower than you would expect at the start of the month.
My assumption is as you’re on a Pro Trial or Free Trial, you might encounter manual allocation issues like this. But once you purchase a plan, the system will handle RU reservation and allocation automatically each month, ensuring your Cloud Robots continue to run without manual intervention.
Ref- Overview - Service licensing
Hope this helps 