I have Catalina macOS version (10.15) installed and still the canvas is not showing up.
The error message I get from Xcode is
Select a scheme that builds a target which contains the current file, or add this file to a target that is built by the current scheme
I filled a Radar FB6123260 please do that also. For me resetting the nvram worked and deleting derived data.
Deleting derived data:
rm -rf ~/Library/Developer/Xcode/DerivedData
Add your new files to the compiled bundles as part of the build phases? Not only does the target need to be set for the file- but it also has to be included in the compiled bundles..
Project -> Target -> Build Phases -> search for the file your are trying to work on