If other devices on the system are working correctly, the most likely reason for this error is that the SUNWdfb package was removed or never installed. Insert the installation CD-ROM, change to the Solaris_2.* directory, and run the following command to install the packages SUNWdfbh and SUNWdfb (for your machine architecture):
pkgadd -d .
If other devices on the system are not working correctly, the system might have a corrupt /devices directory. Halt the system and boot using the -r (reconfigure) option. The system will run fsck(1M) if the /devices file system is corrupted, most likely fixing the problem.