Do a search for ipconfig and find out where it is located. Then navigate to that location to execute.
The error means the file does not exist or could not be found. The system looks in all locations specified by the "Path" environment variable and the current location. If the programs path is not listed in the Path environment variable, you will need to add it or navigate to the location before execution.
The location on my machine is "C:\Windows\System32\ipconfig.exe"
Typing "CD C:\Windows\System32" will take you to that location.
Although looking at the Path string on my machine that location should be in the Path environment variable.
Type "Path" to see all the locations assigned to the Path environment variable.