(1.) Please check the policy of RG, where they accept keywords like: ‘Sex’ or ‘Sexology’?

If you find these terms acceptable by the ResearchGate, please check-

(2.) The file size and format of your book (acceptable size/format by the server of not).

If still you find everything ok, then

(3.) Please check the Internet speed at your end.

But, if ‘1, 2, and 3’ - all are correct then contact the ResearchGate team. I wish they could help you out from this issue.

Hope these will help.