Did you publish a paper in a journal, and this journal is missing in ResearchGate's database? Then see the replies to https://www.researchgate.net/post/How_can_I_add_a_journal_on_ResearchGate2.
Or do you wish to create an account for a journal to upload all papers of that journal? Then see my reply to https://www.researchgate.net/post/I_want_to_open_an_account_of_my_research_journal_on_research_gate.