Not all weapons in Battlefield V will allow you to aim down sights in every position. The Mounted Machine gun is one such weapon that will only allow you to ADS when you are stationary and have something to prop the weapon on or you are on the ground prone.
Most weapons will allow you to ADS at any time, but if you’re using a MMG (Mounted Machine Gun) class weapon which can be found in the Support class options, you’re going to have to get used to either going prone or finding ledges and items to use it with a scope.
The MMG is not a weapon that you should be using for running and gunning as you can’t quickly use a scope or iron sights. Pressing the LT/L2 button will simply have you hip firing with very low accuracy. The flip side is that if you do have a good position with the MMG it can really be a powerful weapon as it’s accurate and powerful.
Aiming down sights with every other weapon in the game will be done with the LT/L2 button.