Have you:
1) opened a command prompt and made sure it has administrator privileges?
2) ensured that you only use the correct command for your version of CompleteFTP
3) Did you stop the Complete FTP Server service before executing the command?
Alternatively please use the below steps:
1) Close CompleteFTP Manager if it's running
2) Stop the CompleteFTP Service in the Windows services console
3) Open a console window with administrator privileges (Run as Administrator)
4) Enter the command: cd “C:\Program Files\Complete FTP\Server” (or for versions prior to 22.0.0, cd “C:\Program Files (x86)\Complete FTP\Server”)
5) Enter the command: cftpconfig /p NEWPASSWORD where NEWPASSWORD is the new admin password
6) Restart the CompleteFTP Service
7) Start CompleteFTP Manager and connect using the new password