Hello,
Do you use a trial or sandbax environment?
If you are using a trial or sandbax environment you are not able to install it.
"It does require a valid production Field Service instance. So you do have to have a valid license production instance of Field Service, and then you can go in through your Office 365 administration, and you can add Resource Scheduling Optimization as an additional service or add-on service on top of it." (Source: Microsoft learning portal)
Regards,
Shayan