尝试更改用户名时，终端会告诉我该进程当前正在使用该用户

引用man usermod：

CAVEATS
   You must make certain that the named user is not executing any 
   processes when this command is being executed
   if the user's numerical user ID, the user's name, or the user's home 
   directory is being changed.  usermod
   checks this on Linux, but only check if the user is logged in 
   according to utmp on other architectures.

因此，您需要确保要重命名的用户未登录。

另外，我注意到您不是以root用户身份运行此程序。要么以root身份运行它，要么以“ sudo usermod”运行。