I’m having the exact same issue as this poster: Can't access existing vault when setting up Obsidian on ios
Exact same. No further notes. I tried the approach recommended in the replies, but it changes nothing.
Where is your vault located in iCloud Drive ? 
Is it “anywhere” in iCloud or is it located in a specific folder titled “Obsidian” ?
The “Obsidian” folder should be looking somewhat like this:
