Yes, you can use the 199A with your EOS 20D. The 199A can be used in Auto exposure mode and there is no need to use a “non-ETTL” mode on the 20D.
The 199A uses a sensor on the front of the flash to automatically adjust the flash power for correct exposures. This older Auto Flash system is fully compatible with almost any camera ever made. (later film camera flash units used a TTL auto system that is not compatible with DSLRs)
All you need to do is set the ISO and aperture on the camera to match the Auto range settings on the flash. Use any shutter speed as long as it is below the sync speed. (1/250 on the 20D)
This is best accomplished using manual mode on the camera. Try ISO 400, f/5.6 and shutter speed 1/125. Set the ISO on the flash and use the Red Auto range. If you want to use f/11 and ISO 400, you would use the Green Auto range.
If you need to, you can download the 199A user manual here: https://www.cameramanuals.org/flashes_meters/speedlite_199a.pdf
