Have succesully created a test class previously.  However, now I have a trigger and a potential test class for that trigger based off some examples but I am not able to do Test | New Run.  I get a message "Add test methods to your test class" but am not sure how to do that in the case of a trigger.  Any help or sugggestions would be appreciated.

Here is the trigger:

trigger UpdateAssetFromSO on Service_Order__c (after update, after delete) {

    List<Id> idAssetList = new List<Id>();
    List<Asset> assetListToUpdate = new List<Asset>();
    
    if(trigger.isUpdate) {
        
        for (Service_Order__c so : trigger.new) {
            
            if(trigger.oldMap.get(so.Id).Total_Reimbursement_Amount__c == so.Total_Reimbursement_Amount__c) continue;
            
            idAssetList.add(so.Asset__c);
        }       
    }
    
    if(trigger.isDelete) {
        
        for (Service_Order__c so : trigger.old) {
                      
            idAssetList.add(so.Asset__c);
        }         
    }
    
    if(idAssetList.isEmpty()) return;
    
    for(Id id : idAssetList) {
        
        Asset asset = new Asset(Id = id);
        asset.SOChanged__c = true;
        
        assetListToUpdate.add(asset);
        
    }
    
    if(assetListToUpdate.size() > 0)
        Database.update(assetListToUpdate);
}

Here is the Test Class (test when the Labor field is updated and when the service order record is deleted):

@isTest(SeeAllData=true)

public class UpdateAssetFromSO_Test {

    public void DoIt(){
    
        //Create Service Orer to work with
        Service_Order__c so = new Service_Order__c(
           Account__c='001j0000016yvCrAAI',
           CurrencyIsoCode='USD-U.S. Dollar',
           Status__c='Open',
           Charge_To__c='11 - Logan',
           Labor_Reimbursement__c = 12.00
           );
           insert so;
        
        //Update Service Order Total Reimbursement Amount is a formula field so we update Labor to make it change.
        so.Labor_Reimbursement__c = 123.00;
        
        //Delete Service Order
        Delete so;
        
    } 
    
}