Most cars will require the engine to be running and the transmission to be warmed up before checking the transmission fluid level.
When you pull the transmission dipstick out it should say on the dipstick whether the engine should be running or not and whether to have the transmission in park or in neutral.
I have a car and a truck and on my car it says to check it in park while on my truck it says to check the transmission fluid with engine running and have the transmission in neutral.
If you have to check the transmission fluid while the car or truck is in neutral make sure you put the parking brake on to prevent the car or truck from running you over.