After replacing transmission pan, my car will start, but won't run?

If I understand this correctly the engine starts and idles fine but when you put it in gear the vehicle will not move?

If this is the case, then more then likely you have a low fluid issue.

I would suggest checking it again and even service it again if needed just to make sure everything is installed correctly like the filter and gasket. If the filter is loose or not installed correctly then it will not pick up the oil and it will not engage the gears.

https://www.2carpros.com/articles/how-to-service-an-automatic-transmission

Please see the info below on how to replace the pan and then check the fluid level. Please run through this material and let us know if you have questions and what you find.

Thanks