As the error says, it looks like you’ve somehow nested a graph into another graph. That’s possible when you tried to create another graph but did so within an existing graph folder (in this case it seems like you’ve created it inside the journals folder?).
As you use GitHub, it could also be an improperly setup repo which caused this nesting.
The best solution now is to manually untangle it, by moving the nested folders out of the journals folder (it should not contain any subfolders).
Yes, Logseq is a local app and all you do by creating a graph is selecting a folder on your device’s disk where to store your notes. It’s best to store your graph in a local folder. Unless you want share a graph between devices or want automated backups, there’s no need to have a syncing solution in place.
However, as you use GitHub I assume you have a need for it?