That message means that the syntax definition for STATS PADJUST was not installed or was installed incorrectly. With older versions of Statistics, restarting it after the installation was necessary in order for the command to be recognized, but that is no longer the case.
Perhaps an OS file permissions problem caused the install to fail. Try starting Statistics using Run As Administrator (on Windows or equivalent on Mac) and doing the install again.
You can see the locations where extensions can be installed by running SHOW EXT. You should find the STATS_PADJUST.xml file in one of those locations.
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