Fill this url with your parameters: {$baseUrl}/rest/prioritycp/1.0/cpconfig?projectId={$projectId}&serviceDeskId={$serviceDeskId}&requestTypeId={$requestTypeId}
Example of localhost with two projects created in Jira, first project created in Jira Core and second with Jira Service Desk for first request type.
If isPriorityScheme will be true and you still will not be able to see less priorities on Create Request screen then please report link that you invoked and returned result to our support.