Is the money agreed upon with an engineer for the preparation of a software project considered a debt that must be repaid, even if the project did not work and was not utilized, and what is the ruling on giving its value in charity on his behalf in the event that he cannot be reached?

Your request for the project from the engineer is either an istisna' (manufacturing contract) or an ijara (leasing/service contract). You must pay the full price of the program or the full wage for his work if he fulfilled the agreed-upon task. The fact that the program did not work on your device does not mean it is defective; the fault might lie with the device or the operating method. You should consult specialists to identify the defect. The engineer is obligated to complete his work, or a deduction should be made from his wage commensurate with the defect. What is due to him must be paid to him. If you despair of reaching him, you are permitted to give charity on his behalf.