That’s really strange, because officially, the bootloader can and should be unlocked.
Teracube 2e (2020) “zirconia” Mod. Serial Number starting with 2020 (ex: 202011T2Exxxxxx)
If you can’t unlock the bootloader using the standard command fastboot flashing unlock, you can try unlocking it using the MTKClient tool. The MediaTek Helio A25 is supported by the MediaTek Flash and Repair Utility “MTKClient.”
Unlock the bootloader and install AOSP 16 on a Teracube 2e (2020) “zirconia”