transmission problem

hello , this is my first thread:
today in my shop came a 728i that have a strange tranny problem which is the car shifts good in revers but when you select drive position the car wont drive and when you shift over to M mode drives good and you can select 1,2,3,4,5 gears upwards and downwards smoothly and it drives great so i hooked it up with a diagnostic device and it said (6a- stall-speed monitoring)??
any ideas for the fix

first check the fluid first.. and if its low then add some.. but if u do that u mite as well just change filter and put new trans fluid in it... while you dont their check all the solenoids and trans connector. and check battery... im having same problem. but i need fluid and filter change and a speed sensor