To Andrea Molyneaux: The usual answers given by RG support team are:

If your journal isn't visible in the drop-down menu, it's not in our database. We're not currently adding journals to our database by request, but this may change in the future.

We're currently making improvements to our journal database, and in the meantime, we're not adding any new titles. Thanks for your patience.

Here is the most recent topic regarding journals at RG Help Center page:

https://help.researchgate.net/hc/en-us/articles/14293139566353-Journals

See my reply to this similar question with a recommendation what to do: https://www.researchgate.net/post/How_can_I_add_the_name_of_the_journal_where_my_article_is_published_when_the_name_of_the_journal_is_automatically_cancelled