VAT not accepted on Vertex website (Urgent)

I was trying to buy on the vertex website but my VAT (BTW for the dutch speakers under us) is not being registered, is there a specific way it has to be inputted to work? Has anyone done this before? I haev tried various times to enter it and have triplechecked if I wrote the number correctly with no fruitful results…

The way I am trying is : BE XXXX XXX XXX or everything together in one block, even added “.” between them to try and get it to work. Can someone tell me what the right way is since I cant seem to find it…