Some vehicles require the engine to be running and the transmission in either park or neutral when checking or adding transmission fluid.
My car says to leave the engine running and have the transmission in park while I check the transmission fluid level and then again my truck says to have the engine running and put the transmission in neutral.
The transmission fluid dipstick should state whether the transmission should be in park or neutral or even your owners manual should say.
But having the engine running while adding transmission fluid is a good idea anyway because it tells you the correct level of transmission fluid in the transmission so you don't overfill the transmission.
Overfilling the transmission with too much transmission fluid can blow out the transmission pump and seals and cause damage to the transmission.