Problem with mergemaster-p

[Zantgo]

I'm trying to upgrade to 9.0 stable, but when I run "mergemaster-p" in single mode, I get the following error message:

*** Creating the temporary root environment in / var / tmp / temproot
mkdir / var / tmp / temproot: Read-only file system

     *** FATAL ERROR: Can not create / var / tmp / temproot

Zantgo