Can't unlock bootloader
Generic answer: The errors in the unlocking process (and especially security errors like yours) generally mean that the device vendor does not allow unlocking. Some device vendors allow it by personally issuing unlock codes (through individual support requests), but it's getting rare nowadays.

Unfortunately the only way is the "old" way, which is to "break into" the device, which usually requires a specific bug on a specific version of Android ROM using a specific program. Since people hardly play with such exploits nowadays and your device seems to be a rather unpopular one your chances to unlock it are sadly very low.

You may try to contact the vendor, if they still exist.