Why I am not able to plunder cash by APC even if it reflects in scouting?

Money can only be taken by the class APC of a raider class player. If you are not a raider then you cannot take it.

If you are using the raider 'class APC' then you will raid money only if there is some available above any protection level that the target base has. If they are a farmer then they will have a high protection level. Other bases may still have some protection.

When you do raid money what you get will depend on what you can carry, the level of money available and the level of other resources that your APC will also take at the same time. If there is a lot of food available for example then you will take mostly food.