Hi now I am getting another error:

UpdateUserQuota: execution of BeforeInsert

caused by: System.DmlException: Insert failed. First exception on row 0; first error: INSUFFICIENT_ACCESS_ON_CROSS_REFERENCE_ENTITY, insufficient access rights on cross-reference id: 005D0000003QtlG: 

Class.QuotaManager.createQuota: line 65, column 1
Class.QuotaManager.getQuota: line 49, column 1
Trigger.UpdateUserQuota: line 55, column 1";s:10:"statusCode";s:36:"CANNOT_INSERT_UPDATE_ACTIVATE_ENTITY";}s:2:"id";N;s:7:"success";b:0;}i:1;O:8:"stdClass":3:{s:6:"errors";O:8:"stdClass":2:{s:7:"message";s:387:"UpdateUserQuota: execution of BeforeInsert

trigger UpdateUserQuota on Opportunity (before update, before insert) {

    if(OpportunityTriggerController.testTrigger != null && OpportunityTriggerController.testTrigger != 'updateUserQuota')
        return;
        
    //if(OpportunityTriggerController.testTrigger == null)
        //return;
    QuotaManager quotaManager = new QuotaManager();
    
    
    List<Id> ownerIds = new List<Id>();
    for(Opportunity opp: trigger.new)
    {
        ownerIds.add(opp.OwnerId);
    }
    
    Map<Id, User> owners = new Map<Id, User>([SELECT Id, Name, Enable_Targets__c FROM User WHERE Id IN :ownerIds]);
    
    List<User_Quota__c> allExistingQuotas = [
            SELECT 
                Id,
                Year__c,
                Quarter__c,
                User__c
            FROM 
                User_Quota__c
    ];
    
    //List<Opportunity> oppsToUpdate = new List<Opportunity>();
    
    for(Opportunity opp: trigger.new)
    {
    if(owners.containsKey(opp.OwnerId)) 
    if(!owners.get(opp.OwnerId).Enable_Targets__c)
            continue;
            
        integer minMonth = 1;
        integer todayMonth = Date.today().month();
        if(todayMonth >= 1 && todayMonth <= 3)
            minMonth = 1;
        else if(todayMonth >= 4 && todayMonth <= 6)
            minMonth = 4;
        else if(todayMonth >= 7 && todayMonth <= 9)
            minMonth = 7;
        else if(todayMonth >= 10 && todayMonth <= 12)
            minMonth = 9;
        
        Date minDate = Date.newInstance(Date.today().year(), minMonth, 1);
        
        if(opp.CloseDate <= minDate && opp.User_Quota__c == null)
        {
            continue;
        }
                
        User_Quota__c quota = quotaManager.getQuota(opp.CloseDate, opp.OwnerId, opp.Owner.Name, allExistingQuotas);  // Line 55
        
        if(quota != null)
        { 
            if(opp.User_Quota__c == null || opp.User_Quota__c != quota.Id)
            {
                opp.User_Quota__c = quota.Id; 
                //oppsToUpdate.add(opp);
            }
        }
        else
        {
            opp.User_Quota__c = null;
        }
    }
    //update oppsToUpdate;
}

and 

public class QuotaManager {
    String used_currency;
    ID opportunity_id;
    
    public QuotaManager()
    {
    }
    
    public User_Quota__c getQuota(Date closeDate, string ownerId, string ownerName, List<User_Quota__c> allExistingQuotas)
    {
        integer month = closeDate.month();
        string year = string.valueOf(closeDate.year());
        
        
       
        
        string quarter = null;     
        if(month >= 1 && month <= 3)
        {
            quarter = 'Q1';
        }
        else if(month >= 4 && month <= 6)
        {
            quarter = 'Q2';
        }
        else if(month >= 7 && month <= 9)
        {
            quarter = 'Q3';
        }
        else if(month >= 10 && month <= 12)
        {
            quarter = 'Q4';
        }
        
        
        User_Quota__c quota = null;
        
        for(User_Quota__c existingQuota: allExistingQuotas)
        {
            if(existingQuota.Year__c == year && existingQuota.Quarter__c == quarter && existingQuota.User__c == ownerId)
            {
                quota = existingQuota;
                break;
            }
        }
        
        if(quota == null)
        {
            quota = this.createQuota(quarter, year, ownerId, ownerName);  // Line 49
            allExistingQuotas.add(quota);
        }

            
        return quota;
    }
    
    public User_Quota__c createQuota(string quarter, string year, string ownerId, string ownerName)
    {
        User_Quota__c quota = new User_Quota__c();
        quota.Year__c = year;
        quota.Quarter__c = quarter;
        quota.User__c = ownerId;
        quota.Target__c = 0;
        quota.Name = 'This text will be replaced';
        insert quota;           // Line 65
        
        return quota;
    }
    
}

If you know what is happening please give me a little help.

Thanks a lot !