Correct:
* You create a Requirement Resource Preference record and set the Preference Type to Restricted.
Incorrect:
* You select Load the Default Filters on the Schedule Board.
* You select the resource in the Restricted Resources field within the Schedule Assistant filter.
Note: Dynamics 365 Field Service, Add preferred resources for a requirement Add preferred resources to a requirement to apply restrictions on resource matches when you use the schedule assistant or Resource Scheduling Optimization for scheduling. The Preference Type field determines how the resources are matched. You can add, change, or remove preferred resources.
1. In Field Service, select the Service area.
2. Under Scheduling, select Resource Requirements.
3. Open a resource requirement, and select Related > Resource Preferences.
4. Select New Requirement Resource Preference.
5. Select a resource and define a preference type:
* Preferred: The schedule assistant shows preferred resources first in the Gantt view if they match all other requirements and filters. It also shows a heart icon next to the name of each preferred resource.
*-> Restricted: The schedule assistant filters out any resources that are marked as restricted. The filter can be removed by using the filter pane.
* Must choose from: The schedule assistant filters results so that they include only these resources. If none of the resources are available, the schedule assistant doesn't return any results. The filter can be removed by using the filter pane.
6. To add another resource, select the dropdown arrow to the right of the Save and Close button, and then select Save & Create New.
7. Select Save & Close.
Reference:
https://learn.microsoft.com/en-us/dynamics365/field-service/resource-preferences