I have written this trigger which adds and opportunity to any new account insertions and account updates (where the account does not have an existing related opportunity).  My current test class provides 76% coverage.  Any advice would be appreciated!

APEX TRIGGER:
trigger AddRelatedRecord on Account(after insert, after update) {

    if (Trigger.isInsert) {
        for (Account a : Trigger.new) {
            Opportunity opp = new Opportunity();
            opp.Name = a.Name + ' Opportunity';
            opp.StageName = 'Prospecting';
            opp.CloseDate = System.today().addMonths(1);
            opp.AccountId = a.Id;
            insert opp;

        }
    }

    if (Trigger.isUpdate) {
        List<Opportunity> oppList = new List<Opportunity>();

        List<Account> accountsWithoutOppsAndGotUpdated = [
                SELECT Id, Name
                FROM Account
                WHERE Id NOT IN (SELECT AccountId FROM Opportunity) AND Id IN :Trigger.new];

 
        for (Account a : accountsWithoutOppsAndGotUpdated) {
            oppList.add(new Opportunity(Name = a.Name + ' Opportunity',
                    StageName = 'Prospecting',
                    CloseDate = System.today().addMonths(1),
                    AccountId = a.Id));
        }
        insert oppList;
    }
}

HERE IS MY CURRENT TEST CLASS:
@IsTest
private class AddRelatedRecordTest {

    @IsTest public static void addRelatedRecordTestonInsert() {

        Account testAcc = new Account(Name = 'My Account Test');
        insert testAcc;
        update testAcc;
        
        System.assert(true, 'Account Updated');

    }

}

Insert failed. First exception on row 0; first error: INSUFFICIENT_ACCESS_ON_CROSS_REFERENCE_ENTITY, insufficient access rights on cross-reference id: []
Error is in expression '{!genDoc2}' in component <apex:commandButton> in page doccontrolgather: Class.DocControlQuery.SavePDF: line 203, column 1
Class.DocControlQuery.genDoc2: line 120, column 1


I need help with this apex code.  I am not a coder :-S