First, ensure that the Maven bin is in your Environmental Variable PATH entry.
If it is, make sure your entries aren't somehow out of order, and that JAVA_HOME is before Path in the list, or any entry that references %JAVA_HOME%. I was getting the same error when I was trying to check my maven version.
I have a few extra path variables that reference %JAVA_HOME%, or a different version of a JDK and Maven was mixed in between. I moved my Maven path entry below my %JAVA_HOME% one and now everything is working when I use Maven from cmd.
But it is Windows, so perhaps my just opening and closing the Environment Variables setting somehow made everything better.