In order to force open locks in Dying Light 2 Stay Human, you’ll first need to upgrade your Lockpick Blueprint to at least level 3. At Lockpick upgrade level 3, you’ll be able to force open Easy locks, but it’ll cost you two Lockpicks every time you do it, so it’s maybe not really worth it. Higher Lockpick upgrade levels will allow you to force open more difficult locks, but it’s always going to be at the expense of some Lockpicks.
To upgrade your Lockpick blueprint, you’ll need some Old World Money and quite a lot of Infected Trophy’s looted from the less common types of Infected. The best place to find these types of Infected is in any dark, indoor location, especially during the day. Some of them are very dangerous though, so stock up on weapons, accessories, and gear before you venture in. Once you have enough Infected Trophies and Old World Money, trade them for Lockpick upgrades at any Craftmaster.
To force open a lock once you’re at the required Lockpick upgrade level, simply approach the door as normal, but instead of pressing Square/X/F, press X/A/H to open the door instantly. The number of Lockpicks this costs you will be displayed on the lock before you force it open.