need a recipe for horchata. is it ok to use powder cinnamon?
Yes, it should be fine. Note that the cinnamon used in Mexico (canela in Spanish) is Ceylon rather than cassia (the common cinnamon in the US). Cassia would still taste good in horchata but noticeably different.
Here's a good recipe: http://www.jeffreymorgenthaler.com/2016/how-to-make-your-own-horchata/