If you followed a link to another thread, JLCPCB rejects board due to empty back solder mask, you saw that it’s impossible to make them understand once they have misunderstood. If you have generated normal gerbers, also B.Mask which is empty, and they don’t understand they should cover the backside with mask, maybe you should send them this message:
Gerber standard uses graphics. The mask layer in gerber is negative, so that where there is graphics in the gerber file, there should be a hole in the mask layer which you apply to the board. If the whole back side of the board must be covered with mask, there is no other logical option than to leave the corresponding gerber file empty. Please tell your customers how to tell you that one side of the board must be covered with mask. If you don’t understand our logic and the logic of gerber files, please tell us your logic so that we can be your customers in the future. If there is some other problem with the files, please tell it clearly. If the only problem was that the back mask gerber layer is empty, why don’t you understand that it must be empty because I want it to be empty, because there is no other way to say “cover the whole board area with mask” in gerber format? Why do you use gerber format if you don’t understand it?
It’s impossible to express how frustrated I am with those manufacturers.