When trying to install a package, the install fails with the error message "The printer driver is unknown".

The driver is included in the client installation package, and has installed successfully (no errors).
You can also create a printer manually with that print driver without any trouble.
This can happen if the driver's name string contains leading or trailing spaces in the .INF file.

Check your printer driver's .INF file to see if it contains leading or trailing spaces inside quotes.

If this is the case, you should be able to work around the problem by removing these spaces in every occurrence throughout the file.
Once this is done, reinstall the driver on the server and make sure your printers and queues are set to use the new driver.  It may be easier to remove the queues, printers, and the driver completely and re-create them to avoid mistakes.

NOTE: You'll need to rebuild the package.