Hi there! I've combed through a bunch of articles trying to find you an answer and from what I've gathered there doesn't seem to be a clear one.. The Common Mobile Payment Issues help article mentions account ineligibility only when trying to buy membership.
I suggest you contact customer support using Issues making a payment. You can also email them directly at firstname.lastname@example.org. If you have access to a computer I'd recommend just purchasing the bonds directly through the website Here as it's cheaper. (no pesky google play store fees) Best of luck!
"Be who you are and say what you feel, because those who mind don't matter, and those who matter don't mind."