Taking a loan without an interest and paying an interest in case of repayment delay
A:
Adding interest to a debt after it was not repaid in time is a clear form of Riba (usury) practiced in Jahiliyyah (pre-Islamic times of ignorance). It is definitely Haram (prohibited), and there are many known pieces of evidence on this.
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.