Got the solution
In Debugger, in Classes\TaxEngineTaxSettlement\createTaxJournal :
taxEngineTaxJournal.initValue();
taxEngineTaxJournal.CurrencyCode = CompanyInfo::standardCurrency();
taxEngineTaxJournal.JournalNum = TaxEngineTaxJournalUtil::getTaxJournalNum();
taxEngineTaxJournal.TransDate = transactionDate;
taxEngineTaxJournal.JournalType = journalType;
taxEngineTaxJournal.insert();
in this line taxEngineTaxJournal.JournalNum = TaxEngineTaxJournalUtil::getTaxJournalNum(); \\from this method it returns values as 'JV-02-0028156' , but the value assigned to taxEngineTaxJournal.JournalNum is only taking 'JV-02-0028' only, journal number already created already so error is throwing. the String size of the journal number is 10, but actual value is 13 characters.