Is it the employee's right to claim an end-of-service bonus if the contract does not stipulate it, knowing that labor law guarantees it? And is the failure to pay it considered an injustice whose sin falls upon the accountant?

If the employment contract stipulates that the employee is not entitled to an end-of-service bonus, then they cannot claim it, and there is no sin upon the accountant or anyone else in that regard. However, if the contract does not stipulate anything, then recourse should be made to common practice and labor law.