The carrier you buy the smartphone from has the option to enforce you to remain with a locked bootloader if they want. In this case, the MetroPCS Oneplus Nord N100 is locked for the first six months after activation, so that is how long the OEM Unlock option will be greyed out in Developer Options. Once the six months have passed, you will be able to enable OEM Unlock.
There’s nothing you can do about it now if you’ve already bought the smartphone. However, it’s certainly one thing to remember in the future before you go shopping. There are different conditions between carriers and sometimes the cheapest smartphones come with preconditions you might not be satisfied with.