Without seeing an image it’s hard to say. So, default trouble shooting steps it is. Select object, tab into edit mode, press A to select all, Press M->By distance(This removes any duplicated vertices. Then Shift+n to calculate normals to outside. now retry your bevel. If it’s still not smooth. just left click, then expand last operation box(Bottom left hand corner of viewport window. It should say bevel on it.) Then make sure that the Shape setting is at 0.5.
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