I am trying to add Raymond Ernest Schrader (deceased) as spouse to my grandmother, Karen Alice Choice (deceased). But I get an error: "Relationship already exists." So my grandma shows up in my tree, but not my grandpa.
As dontiknowyou implied, everything looks fine and dandy to us, because we cannot see your parent or his/her siblings, who are presumably still living, and therefore their profiles are only visible to you.
The symptom you report, of your grandmother showing on your tree views, but not your grandfather, is what would happen if your parent was entered only as a child of your grandmother, separately from your grandmother's relationship to your grandfather. That is, the relationships would look something like this:
(Screenshot is from the beta site, not the real one.) If I then clicked the topmost "Add Spouse" button on Grandma's profile and tried to add Grandpa, I'd get the "Relationship Already Exists" message:
If, however, I click the "Add Spouse" in the box above single-parent Grandma (the one visible in the first screenshot above), or if I use the pencil-stub-and-box icon to the right of Testy Parent's name and choose "Add Father", then I can connect Grandpa to Parent and get rid of the extra relationship:
There are other situations where one way results in "already exists" while a different way works fine, but I can't remember the details well enough to re-create such a scenario in beta. In any case, the key to your problem is likely not in Grandma's relationship to Grandpa, but in your parent's relationships to Grandma and Grandpa.
What PIDs are you looking at? I see KN7J-V2X and LZJL-2PD.
Thank you! By going to my parent and changing the "preferred" parent and sibling relationships, I can now see both grandparents on the tree.