@oliverehmer Welcome to RStudio Community!
This appears to be a new option in R 3.6.
Do you ever use system.file() to access files in your package? Reading the announcement of the Staged Install check, this is one of the common sources of the error.
The announcement also mentions that you can opt out of the check by adding StagedInstall: no to your DESCRIPTION file. Maybe that will work better than the flag --no-staged-install.
And if you need a quick fix, you could always downgrade to R 3.5 until you figure out how to handle the new R 3.6 behavior.
Resources:
- RStudio Community: Error installing R package from local file: hard-coded installation path.
- Stack Overflow: Package build fails on R 3.6.0 (devel) but not on 3.5.5 (release)