Here is the test class
global class OppPropExtension {
    WebService static string CreateProposal(string OppId) {
    
    string retVal = '';
    

    
    Opportunity Opp = new Opportunity();
    Opp = [select Proposal_Revision_Reason__c, Approved_by_Practice_Area__c, Manager_Approved__c, B_P_Resources_Required__c,
                       Opportunity_ID__c,  
                       Bid_Due__c,
                       Deal_Type__c,
                       Id,
                       LOE_Required__c,     
                       BOM_Required__c,  
                       Subcontractor__c,
                       Contract_Vehicle__c,   
                       OwnerID,
                       Practice_Area_Reported__c 
                       from Opportunity where id =:OppId] ;

    List <ProposalRequest__c> PropToInsert = new List <ProposalRequest__c> ();
    
    //for (Opportunity o : Opps) {
        
        if(
            //(Opp.Proposal_Revision_Reason__c.length() > 0 ) &&
            !String.isEmpty(Opp.Proposal_Revision_Reason__c) &&
            (Opp.Approved_by_Practice_Area__c == true)     &&
            (Opp.Manager_Approved__c == true )             
            )
            {
         
                    // check if opportunity that is being inserted meets the criteria
                    //if(Opp.B_P_Resources_Required__c == true) {
                        
                        ProposalRequest__c p = new ProposalRequest__c (); //instantiate the object to put values for future record
                        // now map opportunity fields to new proposal request
                        p.Name = Opp.Opportunity_ID__c + ':' + Opp.Deal_Type__c; 
                        p.Bid_Due__c = Opp.Bid_Due__c;
                        p.Deal_Type__c = Opp.Deal_Type__c;
                        p.Opportunity__c = Opp.Id;
                        p.LOE_Required__c = Opp.LOE_Required__c;     
                        p.BOM_Required__c = Opp.BOM_Required__c;  
                        p.Subcontractor__c = Opp.Subcontractor__c;
                        p.Contract_Vehicle__c = Opp.Contract_Vehicle__c;   
                        p.OwnerID = Opp.OwnerID;
                        p.Proposal_Revision_Reason__c = Opp.Proposal_Revision_Reason__c;
                        p.Technology_PA__c = Opp.Practice_Area_Reported__c;                      
                         //add this new object to the list that will be inserted.       
                        PropToInsert.add(p);       
                   // }
                   
                    // dml operations might cause an error, so you need to catch it with try/catch block.
                    try {
                        insert PropToInsert; 
                    } catch (system.Dmlexception e) {
                        system.debug (e);
                        return retVal;
                    }
                    
                     retVal = 'Proposal Request created successfully'; 
                     return retVal;
            
            }
            else
            {
                 retVal = 'Opportunity must be approved and revision reason selected before requesting a proposal revision'; 
                 return retVal;
            
            }      
   }  
 }

And here is the trigger 

trigger OppCreateProposalReq on Opportunity (after update) { 
    List <ProposalRequest__c> configToInsert = new List <ProposalRequest__c> ();
    
    for (Opportunity o : Trigger.new) {
        
        if(
            (o.Approved_by_Practice_Area__c == true && trigger.oldMap.get(o.id).Approved_by_Practice_Area__c == false) 
            &&
            (o.Manager_Approved__c == true && trigger.oldMap.get(o.id).Manager_Approved__c == false)
            )
            {
         
                    // check if opportunity that is being inserted meets the criteria
                    if(o.B_P_Resources_Required__c == true) {
                        
                        ProposalRequest__c p = new ProposalRequest__c (); //instantiate the object to put values for future record
                        // now map opportunity fields to new proposal request
                        p.Name = o.Opportunity_ID__c + ':' + o.Deal_Type__c; 
                        p.Bid_Due__c = o.Bid_Due__c;
                        p.Deal_Type__c = o.Deal_Type__c;
                        p.Opportunity__c = o.Id;
                        p.LOE_Required__c = o.LOE_Required__c;     
                        p.BOM_Required__c = o.BOM_Required__c;  
                        p.Subcontractor__c = o.Subcontractor__c;
                        p.Contract_Vehicle__c = o.Contract_Vehicle__c;   
                        p.OwnerID = o.OwnerID;
                        p.Proposal_Revision_Reason__c = o.Proposal_Revision_Reason__c;
                        p.Technology_PA__c = o.Practice_Area_Reported__c;                      
                         //add this new object to the list that will be inserted.       
                        configToInsert.add(p);       
                    }
            
            }
    
    //once loop is done, you need to insert new records in SF
    // dml operations might cause an error, so you need to catch it with try/catch block.
            try {
                insert configToInsert; 
            } catch (system.Dmlexception e) {
                system.debug (e);
            }
    }
}

Any help will be appreciated. Thanks