Just in case enabling it at developer settings isn’t the reason.
Have you seen this thread:
There is a FP-page with a tool to create the verification code. And on this page (in the source code, there is an error-message: "data-error-disabled=“Unlocking the bootloader is disabled for this device. Sorry about that.” So maybe your phone is one of the devices, where unlocking is disabled? Is it a company phone?