Trigger:

trigger Rfleet_DisableAttachements on Attachment(before insert,before delete,after update) {
Id devRecordTypeId = Schema.SObjectType.Opportunity.getRecordTypeInfosByName().get('RFLEET-OPP-DCVF-RT').getRecordTypeId();
Id devRecordTypeId1 = Schema.SObjectType.Opportunity.getRecordTypeInfosByName().get('DC-VF Read only RT').getRecordTypeId();
if(trigger.isinsert && trigger.isbefore){
 for (Attachment att:Trigger.new){
        
            String parentObjId = att.ParentId;     //It will get profitability obj id.  
            
           list<opportunity> pid= [select Rfleet_locked__c,recordtypeid from Opportunity where Rfleet_locked__c=true ];// it get list of opp locked records.
               list<Rfleet_Profitability__c> ids=[Select Rfleet_opportunity__c from Rfleet_Profitability__c where id=:att.ParentId];
            for(Rfleet_Profitability__c pro:ids){
                for(opportunity opp:pid){    
                    if(parentObjId.startsWith('a3r') && opp.Rfleet_locked__c==true && opp.id==pro.Rfleet_opportunity__c && opp.recordtypeid==devRecordTypeId )//a3r is the starting sting in ID for all Profitability
                    {
                    att.addError('Since this opportunity was submitted for approval, you are not allowed to add a new attachment.');
                    }
                    else if(parentObjId.startsWith('a3r') && opp.Rfleet_locked__c==true && opp.id==pro.Rfleet_opportunity__c && opp.recordtypeid==devRecordTypeId1)
                    {
                    att.addError('Since this opportunity was closed, you are not allowed to add a new attachment.');
                    }
                }
            }
    }
 }
 if(trigger.isdelete && trigger.isbefore){
     for (Attachment att:Trigger.old){
      String parentObjId = att.ParentId; 
      list<opportunity> pid= [select Rfleet_locked__c,recordtypeid from Opportunity where Rfleet_locked__c=true ];// it get list of opp locked records.
               list<Rfleet_Profitability__c> ids=[Select Rfleet_opportunity__c from Rfleet_Profitability__c where id=:att.ParentId];
            for(Rfleet_Profitability__c pro:ids){
                for(opportunity opp:pid){    
                    if(parentObjId.startsWith('a3r') && opp.Rfleet_locked__c==true && opp.id==pro.Rfleet_opportunity__c && opp.recordtypeid==devRecordTypeId)//a3r is the starting sting in ID for all Profitability
                    {
                    att.addError('Since this opportunity was submitted for approval, you are not allowed to delete the attachment.');
                    }
                    else if(parentObjId.startsWith('a3r') && opp.Rfleet_locked__c==true && opp.id==pro.Rfleet_opportunity__c && opp.recordtypeid==devRecordTypeId1)
                    {
                    att.addError('Since this opportunity was closed, you are not allowed to delete the attachment.');
                    }
                }
            }
     }
 
 }
 if(trigger.isupdate && trigger.isafter){
     for (Attachment att:Trigger.new){
      String parentObjId = att.ParentId; 
      list<opportunity> pid= [select Rfleet_locked__c,recordtypeid from Opportunity where Rfleet_locked__c=true ];// it get list of opp locked records.
               list<Rfleet_Profitability__c> ids=[Select Rfleet_opportunity__c from Rfleet_Profitability__c where id=:att.ParentId];
            for(Rfleet_Profitability__c pro:ids){
                for(opportunity opp:pid){    
                    if(parentObjId.startsWith('a3r') && opp.Rfleet_locked__c==true && opp.id==pro.Rfleet_opportunity__c && opp.recordtypeid==devRecordTypeId)//a3r is the starting sting in ID for all Profitability
                    {
                    att.addError('Since this opportunity was submitted for approval, you are not allowed to update the attachment.');
                    }
                    else if(parentObjId.startsWith('a3r') && opp.Rfleet_locked__c==true && opp.id==pro.Rfleet_opportunity__c && opp.recordtypeid==devRecordTypeId1)
                    {
                    att.addError('Since this opportunity was closed, you are not allowed to update the attachment.');
                    }
                }
            }
     }
 
 }
 }

test class:
@isTest
public class Rfleet_DisableAttachements_Test {
static testMethod void  Rfleet_DisableAttachements(){

Account acc = new Account(Name='vfvh',montant__c=0.3);
insert acc;
acc.name='hjghg';
update acc;
  
Opportunity opp=new Opportunity();
RecordType rt = [select Id from RecordType where Name = 'RFLEET-OPP-DCVF-RT' and SobjectType = 'opportunity' LIMIT 1];
opp.RecordTypeId = rt.id; 
opp.Rfleet_locked__c = false;
opp.name='fdf';
opp.stagename='Draft';
opp.closedate=System.Today();
insert opp;
opp.name='save';

update opp;


Rfleet_Profitability__c Pro=new Rfleet_Profitability__c();
Pro.name='Testpro';
Pro.Rfleet_opportunity__c=opp.id;


Attachment attach=new Attachment(); 
attach.Name='TestAtt'; 
Blob bodyBlob=Blob.valueOf('Unit Test Attachment Body'); 
attach.body=bodyBlob; 
attach.parentId=opp.Id; 
insert attach; 
       
List<Attachment> attachments=[select id, name from Attachment where parent.id=:opp.id];
System.assertEquals(1, attachments.size());
 attach = [SELECT Id, name from Attachment where parent.id=:opp.id];
 delete attach;  
       
     }
    
 }