I was following Sony's official guide for unlocking the bootloader. However, upon entering
*#*#7378423#*#* into the phone App and navigating to
Service Info -> Configuration under the Rooting Status category, it said
Bootloader unlock allowed: No
I tried Googling on how to solve the issue, but I mostly hit dead-ends. A few of the guides claimed that flashing a stock ROM would work, but it was often met with negative feedback from other readers. There are apparently some paid services that do this, too, but I'm not in the position to resort to them. Some people said it's virtually impossible to do it.
The main reason why I want to flash the ROM is because my operator/carrier (T-Mobile) has loaded the phone with bloatware and there's this one application that just keeps giving me grief with notifications. Also, I've grown very familiar with Cyanogenmod with my previous phone, and I miss it!