Sencha docs によると、可能な解決策は次のとおりです。
Upon running sencha, if there is an error message appears saying "sencha: command not found" on OS X / Linux or "'sencha' is not recognized as an internal or external command, operable program or batch file." on Windows, follow these steps to troubleshoot:
Close all existing terminal / command prompt windows and re-open them.
Make sure that Sencha SDK Tools is properly installed:
The installation directory exists. By default, the installation path is /Applications/SenchaSDKTools-{version} on OS X, /opt/SenchaSDKTools-{version} on Linux, and C:\Program Files\SenchaSDKTools-{version}
The path to SDK Tools directory is prepended to your system's PATH environment variable. From the terminal, run echo $PATH (echo %PATH% on Windows). The SDK Tools directory should be displayed in part of the output. If this is not the case, add it to your PATH manually.
The environment variable SENCHA_SDK_TOOLS_{version} is set, with the value being the absolute path to the installation directory mentioned above. For example: If the installed version is '2.0.0-beta2', a SENCHA_SDK_TOOLS_2_0_0_BETA2 must be set. From the terminal, run echo $SENCHA_SDK_TOOLS_2_0_0_BETA2 (echo %SENCHA_SDK_TOOLS_2_0_0_BETA2% on Windows). If the output is empty, set the environment variable manually.
Please check Troubleshooting heading in below link
http://docs.sencha.com/touch/2-0/#!/guide/command
EDIT:
Check these answers on SO:
[WARN] The current working directory (C:\xampp\htdocs\sencha folder) is not a recognized Sencha SDK or application folder.
SenchaSDKTools-2.0.0-beta3-windows Command not working