can you still use powdered milk that separate when dissolved in water?

Providing you mixed the powder and the water in the proportions given in the package instructions, you can use it, but it will taste thin.  You need to stir or shake it a lot to get the powder to dissolve completely.  It also helps to use warm water.

This isn't really a question about Chemistry.  You'd likely get better answers by moving it to some category like:

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