Can I say I'm undecided yet in my statement of purpose ? (PhD in math)
Generally in the US you don't need to choose an advisor when you start Doctoral study. Therefore, having a firm idea about a concentration, research focus, is less important than in some other places. But giving the admissions people some idea will give the a better way to think about how you might fit in. 

What I would do, in your case, is to say you are undecided, but currently focused on X and Y, where X and Y are a couple of things you know a lot about. It narrows the focus a bit without limiting you in any way. But, having an MS means that you will want to get started on research pretty soon after arrival and so will need to choose an advisor, unless you want to risk having one chosen for you (a bad idea, generally). 

However, taking a deep dive into some narrow subfield of math doesn't limit your choices for ever. You will need to focus until you earn the degree, but then the view can open again for you if you want. 

It is probably ok to be a bit undecided as long as you aren't chaotic random. 



Note that this is a US specific answer. It wouldn't be the same in some other places.