When checking the fluid level are you following the manufacturer guidelines . The engine needs to be running at operating temp and the vehicle needs to be running . If not you wont have enough fluid in the trans . Did you use the right trans fluid for the vehicle. May transmissions have been ruined by the wrong trans fluid . Did you clear the code after the fluid was replaced . It may take some time for the valve to clear after the new fluid and filter are done . There is also, in this codes case , a possibility its not a transmission problem but a idle problem . If the ECM does not detect the proper rpm it doesn't shift and if theres a problem with the throttle body it wont shift properly, just something to check . One last thing to check , You mentioned your loosing coolant . does your transmission have its own cooler or does it share a cooler in the rad? In other words is there lines coming from the trans into the side of the rad , if so check the rad fluid for signs of oil and the trans fluid for signs of coolant. As for the coolant leak do a pressure test to see if it holds pressure. put paper down after you fill it to see if theres coolant anywhere . a leakdown test will help here as well to determine if you have a head gasket problem . Are you experiencing overheating ? are you seeing any white smoke from the exhaust. Just some things to look for . Hope this helps