Hi Daniela. That usually happens when your HiPoly model is poking through your cage (the LoPoly one).
One way to try to fix this is to increase “Ray Distance” in the properties panel under Bake -> Selected to Active.
Another way is to select the cage, go into Edit mode, press 3 to edit faces, select all, press Alt-S and move the mouse to move all faces outwards a little, and try to bake again.
If you don’t want to alter the shape of the lopoly model, make a duplicate of it and use the duplicate as your cage, so you can change it and still have the original, unaltered lopoly model.
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