Thank you for your post!
As beverly suggested to you, I'm going to move this thread from the FileMaker Community Feedback Space, which is specifically for input on the Community itself, to the Discussions Space where you should receive more views and potentially more feedback on this topic!
If you try to put a portal on a layout and the desired Table Occurrence appears in the "Unrelated" section, then you do not have a relationship linking the layout's table occurrence to the table occurrence you want to select for the portal.
You may find this tutorial thread helpful:
Thanks for sending the tutorial, but it only addresses the basic process, not WHY DOES THIS PROCESS SIMPLY NOT WORK sometimes. The problem is I did create a table to table relationship. Sometimes they work and sometimes they don't. Many times I have to scrub the relationship and try again, and sometimes it will work and a 2nd or 3rd try. Other times, the relationship simply refuses to be recognized within a portal. I have tried a hundred ways to do it (indexed? text vs number? includes a calc? etc etc) , and have been unable to find what causes relationships to be ignored.
Never had a problem with this ever over many years of work. Thus, I am guessing that there is a mismatch between the table occurrence specified for the layout and the portal such that the relationship you think that you specified is not available for the current layout due to either picking the wrong table occurrence for the layout or the wrong table occurrence for the portal.
If you have an example of this please upload a screen shot of the relationships and tell us which table occurrence is specified for the layout and which you want to specify for the portal, but can't.
It may be possible to encounter a problem when the table occurrence has an external data source from a file that is not currently open, but normally, attempts to reference that table occurrence automatically opens the needed file in the background so unless you get something like a password dialog and cancel it instead of entering the needed credentials, it shouldn't cause a problem for you.