See https://help.researchgate.net/hc/en-us/articles/14293139566353-Journals for explanations why some journals are missing. Unfortunately, it is not possible to add a journal to RG's database "by hand". In case of a missing journal in this database, I add the bibliographic data to the abstract field (like, e.g., in https://www.researchgate.net/publication/344474227 and https://www.researchgate.net/publication/268925009).
The reason for it is that the database of ResearchGate includes only journals that are indexed in well-known databases such as Web of Science (Clarivate) and Scopus (Elsevier). If your journal is not indexed in those databases (e.g. predatory publication/journal) or not a well-established preprint server such as arXiv or Qeios, then this might be the reason for ResearchGate not including your journal.
If you're unable to find the title of the journal in which you published your article on ResearchGate, it could be due to several reasons. The journal may not be included in ResearchGate's database, which is common for newer or less widely indexed journals. There might also be slight variations or misspellings in the journal's name, so ensure you are searching with the correct and full name. Technical issues or glitches can sometimes prevent certain entries from appearing in search results. Additionally, ResearchGate might not index all journals, especially those not widely recognized or indexed in major databases. If you still cannot find the journal, you can manually add its details while uploading your publication by typing in the journal's name manually. This ensures your publication is correctly attributed, even if the journal is not pre-listed in the ResearchGate database. If problems persist, consider reaching out to ResearchGate support for further assistance.