You cannot input less than 8 ASCII character as it is defined on 802.11 standard
What you can do is to use more than 8 ASCII character.
You can also see the protocol's implementation on the source code of the wpa_passphrase
if (os_strlen(passphrase) < 8 || os_strlen(passphrase) > 63) {
printf("Passphrase must be 8..63 characters\n");
return 1;
}
Btw, wpa_passphrase is used to generate password phrase, not to connect a WLAN. And the passphrase is used on wpa_supplicant.conf which is a tool that I would recommend you to use, for more info click on[wpa_supplicant][1].
Alternatively you can use iw tool or the deprecated iwconfig. However, these two tools work for only WEP encryption that is phasing out (and of course open network (without encryption)). You can not connect to an AP that use WPA or WPA2 encryption