"If taken as the grounds of feminist theory or politics, these "effects" of gender hierarchy and compulsory heterosexuality are not only misdescribed as foundations, but the signifying practices that enable this metaleptic misdescription remain outside the purview of a feminist critique of gender relations."
In this sentence, Judith Butler is saying that since gender is socially, and even the idea of two different biological sexes are socially constructed, feminism (when founded on the acceptance of women as a category that does not need to be critiqued or questioned) is founded on a false construction. More importantly, the false construction of these categories of "men" and "women" might be the root of all of the hierarchies and power disparities that feminism is supposed to critique! As she says later "this foundationalism...presumes, fixes, and constrains the very "subjects" that it hopes to represent and liberate." Between those two phrases, she means to say that accepting the gender binary and the predicursive existence of "women" is not grounds for a very helpful or constructive "feminism."