Reading the code and the test class, it should work.

i have simulated that code in a dev environment and I can achieve 86% coverage.

I suspect the issue is with the Campaign not being returned in you code.  The campaign will exist as a record in the live environment, and therefore is not visible to the test class.
So, I would adjust as follows: (note that I don't think Ids should ever be hardcoded into apex class, it makes them less portable.)
Class:

/** 
* Created by andrewg on 31/07/2020. 
*/ 
public with sharing class OppPrintCampaign {
    public static void InsertOpp(List<Opportunity> lstopp,Map<Id,Opportunity> mapIdToSatge){
        Set<Id> setOfAccId = new Set<Id>();
        List<Account> lstupdateacc = new List<Account>();
//        List<Opportunity> lstofClosedLostOpp = new List<Opportunity>();
//        Campaign objcamp = [Select Id,name from campaign where Id = '7012v000002KBp9'];
        Campaign objcamp = [SELECT Id,Name FROM Campaign WHERE Name = 'Some test'];

        for(Opportunity objopp:lstopp){
            if(objopp.AccountId != null && objopp.StageName == 'Closed Lost' && objopp.StageName != mapIdToSatge.get(objopp.Id).StageName){
                setOfAccId.add(objopp.AccountId);
            }
        }
        //next element does what exactly?  
//makes another query to confirm my first code was correct? removed to improve coverage
        /*
        System.debug('-----setOfAccId-----'+setOfAccId);
        for(Opportunity objopp :[SELECT Id,Name ,StageName ,AccountId FROM Opportunity WHERE AccountId IN:setOfAccId]){
            if(objopp.StageName != 'Closed Lost'){
                lstofClosedLostOpp.add(objopp);
                break;
            }
        }*/
        if(setOfAccId.size() > 0){
            for(Account objacc:[SELECT Id,Name FROM Account WHERE Id IN:setOfAccId]){
                objacc.Site = objcamp.Name;
                lstupdateacc.add(objacc);
            }
        }

        System.debug('-----lstupdateacc-----'+lstupdateacc);
        update lstupdateacc;
    }
}

Test class
Since we appear worried about checking if the value was updated correctly, lets also do a negative test where the record is not updated.
 

/**
 * Created by andrewg on 31/07/2020.
 */

@IsTest
private class oppPrintCampaignTest {
    @TestSetup
    static void setupData(){
        Account objacc = new Account();
        objacc.Name = 'Test';
        objacc.Rating = 'Cold';
        insert objacc;

        Opportunity objopp = new Opportunity();
        objopp.Name = 'Test opp';
        objopp.CloseDate = System.today();
        objopp.AccountId = objacc.Id;
        objopp.StageName = 'New';
        insert objopp;

        Campaign camp = new Campaign(
                Name = 'Some test',
                IsActive = true
        );
        insert camp;
    }

    @IsTest
    static void testFieldUpdate(){
        Opportunity objopp = [SELECT Id, Name, StageName FROM Opportunity WHERE Name = 'Test opp'];

        //force an update so the code runs
        objopp.StageName = 'Closed Lost';
        update objopp;

        Account updatedAccount = [SELECT Id, Site FROM Account WHERE Name = 'Test'];
        System.assertEquals('Some test',updatedAccount.Site);
    }

    @IsTest
    static void testFieldNonUpdate(){
        Opportunity objopp = [SELECT Id, Name, StageName FROM Opportunity WHERE Name = 'Test opp'];

        //force an update so the code runs
        objopp.StageName = 'Closed Won';
        update objopp;

        Account updatedAccount = [SELECT Id, Site FROM Account WHERE Name = 'Test'];
        System.assertEquals(null,updatedAccount.Site);

    }

}

Note, we also use the @TestSetup convention to setup the test data once.
We have also removed the use of testMethod as that has been deprecated. 
ref: https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_classes_annotation_isTest.htm

The above will give 100% coverage and if your Trigger structure looks like below, it would 100% coverage also:

/**
 * Created by andrewg on 31/07/2020.
 */

trigger oppTrigger on Opportunity (after update) {
    System.debug('DEBUG - In Trigger - oppTrigger');

    if ( Trigger.isAfter && Trigger.isUpdate ) {
        System.debug('In Trigger - about to call on after update');
        OppPrintCampaign.InsertOpp(Trigger.New, Trigger.OldMap);
    }


}

Note also, i would change the method name, as insertOpp is not really a clear name for what the method is doing.
For a naming convention, my handler would be named opportunityTriggerHandler - some may say not very original, but it means I can add other logic (methods) to the class but the class purpose is still quite obvious.
I would then change the method name to something like setAccountCampaignReference - yes, a little unwieldy, but it is quite obvious what the method is about to do.

That way I could read the in the trigger as:

if ( Trigger.isAfter && Trigger.isUpdate ) {
        System.debug('In Trigger - about to call on after update');
        opportunityTriggerHandler.setAccountCampaignReference (Trigger.New, Trigger.OldMap);
}

And i would have a clear idea of what was happening without necessarily going to the handler class to find out.


Regards
Andrew