Can't Flash Recovery Because of S-ON?

I have a T-Mobile HTC One M9 with TWRP installed, running custom builds of CM12.1. I tried to flash the CM recovery via fastboot (fastboot flash recovery recovery.img) just cuz I wanted to see if my recovery builds work as well, but when I do, it fails saying that it can't flash that partition due to S-ON... This is obviously not the case seeing as I have TWRP installed already, and I used fastboot to install it... I'm thoroughly confused.

Any help would be GREATLY appreciated, thank you.