When ww2 began the belgian government decleared themselves neutral. Therfor, acording to the Hague conference of 1907, no participating nation can send troops into their country and doing this would be violating their neutrality. Similarly the neutral country, in this case Belgium, could not allow other nations to have troops on their land as this would be the same as supporting that side. They would therefor not be neutral anymore. So the germans could not ask to get military access because this would practicaly be asking for belgium to support Grrmany and give up their neutrality