ah, sorry for the bumps 
let’s try the following:
- open your Particle project
- run the
Particle: Install Local Compiler Toolchaincommand (docs) - select
deviceOS@1.4.0-rc.1and opt to “reinstall” if prompted - wait for the install to complete - progress should be displayed in the status bar and a “working” notification should be shown bottom-right. upon completion, a notification with a list of what was installed will be shown bottom-right (click the “bell” icon if you don’t see 'em)
- within your project, delete the
./targetdirectory if it’s available - run the
Particle: Compile application & DeviceOS (local)task (from VSCode’s main menu, select “Terminal > Run Task…” then select from the list shown)
if that doesn’t help, please run Particle: Audit Environment and share the non-sensitive bits here 