Log in to your Google Voice acct and add the phone number of your phone to the list.
The on line acct has to have one active number on it, does not have to be the number to your phone just any number that the Google Voice servers can send a text to.
I have two phones set up with GoogleVoice/GrooVeIP to get free voip phone service. The only number these two phones have are the GV number and everything works, but GV could not verify the phones until it confirmed a phone. ANY phone. So I added one of my never used TracFone numbers that I can't even install Android apps on and let GV send a text to that phone, once done the acct/service was confirmed and I can send and receive calls and text to both Android phones on the GV number, you have to have some/any number on the GV acct that can receive text.
If thats not the issue you are having, is it that you have the app on your phone but don't have a GV acct set up on line? You must have a GV acct first before the app will work on your phone, and remember that after all that is said and done, that is you have gone on line and made your GV acct GV will try to call your phone, it will put a box on the screen that has two numbers on it, when GV calls your phone, listen to the prompts and when it tels you to you need to enter those two numbers, if you don't it can't confirm your acct.
But the big thing is the app is no good unless you have the GV acct already set up on line.