Xiaomi Mi5s Plus: Can't Update Via Ota

[Marcgise]

I have the Global ROM of the Xiaomi mi5s plus. I can't update the ROM via OTA since I unlocked the bootloader (at least I think that is since I did it).

Is it possible to lock the bootloader again? Is this the reason why I can't update it?

I tried with "fastboot oem lock", but it didn't work.

My version of the ROM is 8.0.6.0.0 | Stable, and I'm always getting the following error:
Can't download update: Wait a minute or two and try again.

Thanks,