Hopefully you've checked the fluid? And hopefully its at the right lever, clean & red, and lacking any foul odor? How many miles are on it, and what kind of service has the transmission seen over its lifetime?
You'll probably need someone with a better code reader/scanner. If there is a warning light then there are codes. A lot of code readers only pick up generic OBD-2 codes.
I'd suggest just finding your best local transmission shop and asking them to look at it. Some do this for free. Others might charge an hour's labor. If you end up with some codes & a diagnosis you can always post the codes & the story here for comment.