While many people may suggest that it is not a good idea to put a countertop microwave in a cabinet, yes, you can always place/ put a countertop microwave in a cabinet. You need to follow some precautionary measures before that; like ensuring that there is enough space for keeping the microwave, it is not at a closed space, there is enough ventilation and the place is not consisted. If the place is upto these measures, you can put the countertop microwave in the cabinet without any worries.