Supposed to is the conventional spelling of the adjectival phrase meaning required to, expected to, or allowed to. But suppose to is common enough to be considered acceptable at least in informal writing, and it does not cause confusion. For example, no English speaker would misunderstand these sentences simply because they use suppose to instead of supposed to:
It does what it’s suppose to do at least as well as any other stick I’ve used. [Wired]
Capps has been a proven closer and was suppose to be the early option if Nathan didn’t seem ready. [Fergus Falls Daily Democrat]
Be aware, however, that some readers will consider suppose to incorrect. In formal contexts or where you need to be taken seriously, supposed to is the safer choice. Elsewhere, using suppose to is not a serious error.
A nurse who was supposed to be caring for elderly people got intoxicated on painkillers. [Scotsman]
It was supposed to be a number-packed speech about budgeting and spending and all manner of fiscal gobbledygook. [Politico]
As a journalist, I know I’m supposed to be objective. [Edmonton Journal (article now offline)]
I don’t even know what a ylang ylang is, let alone what it is supposed to smell like. [Sydney Morning Herald]