I want to ask about Nigeria visa issue maybe is...

Note that the Legal Section at Jawabkom website only provides legal consultations, and doesn't have any relation with any entity whether governmental or nongovernmental.
In regard to permitting Nigerian citizens from obtaining the visa, work is a matter subject to the administrative decisions, internal decisions organized by the UAE government which did not announce a clear decision for this matter.
You can contact the General Administration of Residence in order to find out the reason for this via the link: