A job was posted with a language variant that, in my opinion, does not exist. Is this allowed?

According to point 6 of the ProZ.com guiding principles, job posters are the ones who define the way they present their needs to the community. Academic consideration on the existence or not of a given language variant are not relevant here, this is simply a channel for an outsourcer to further specify a translation request. Information in this field is not used to filter translators, so nobody will be excluded from the job.