Rim brake on front and disk brake on rear
With no disk brake mounting lugs on the frame you cannot mount disc brakes. There are bolt on adapters but I personally would not trust them. Additionally you would need a disc compatible hub which in reality means a whole new rear wheel.

You should check that that the wheel rims are compatible with rim brakes. There appear to be reflective stickers where the braking surface should be.

I always advise buying the bike you want, not a fundamentally different one hoping you can convert it. The easiest path for you might actually be to sell this bike and buy a single speed with front and rear rim brakes.