ABS light on Daewoo Nubira of 99.

Hello everyone, I would appreciate it if someone could help me with an ABS problem on a 1999 Daewoo Nubira. The system isn't working; when I start the car, the ABS light stays on and I can hear the relay next to the fuse box clicking. I replaced the relay in case it was the problem, but it wasn't. I've checked the fuses, the wheel speed sensors, and the sprockets visually and cleaned them. I don't have a diagnostic tool, which is obviously a problem. Has anyone had a similar experience or can offer any advice on this vehicle? Thank you very much, and best regards.