Can't Update factory unlocked att HTC One m7

I bought an unlocked ATT HTC One m7 and it has android 4.4.2 loaded onto it. I want to update it to the lollipop update, but it's on MetroPCS so I can't use ATT software update. Tried to use RUU on HTC website but I keep getting ERROR 155:. What do I do? Thanks!

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