all sinks and tubs draining from upstairs is draining but when you flush upstairs toliets running out downstairs toliet. The washing machine down stairs is not backing up. On septic tank system

Assuming your tank isn't full, I would say you have a partial blockage in the mainline, past the downstairs toilet. You will need to isolate the blockage. Since the washing machine isn't backing up, it's possible the blockage is between the toilet and the washing machine.

Try running a snake from the toilet and see if you can clear anything out. If that doesn't help, find the sewer cleanout (it will be outside close to the house). Open that up, then flush the toilets upstairs. If water flows well past the cleanout, then the blockage is inside the house. If the water flows OUT of the cleanout, then the blockage is between there and the tank. That could also indicate your tank is full...