可能的原因包括不允许空密码，登录时间限制，或强制的策略限制。

Possible reasons are blank passwords not allowed, logon hour restrictions, or a policy restriction has been enforced.

youdao

一旦有密码系统安装，斯托曼就将它破解，然后设为空密码并发邮件告诉同事们。

When a password system was installed, Stallman broke it down, resetting passwords to null strings, then sent users messages informing them of the removal of the password system.

youdao

对于主机密钥，您可能不需要密钥密码，所以您可以按Return使用空密码。

For host keys, you probably don't want a passphrase for the key, so you can just press Return to use a blank (empty) passphrase.

youdao