I responded to your other inquiry. Here it is again....
First, do you have a RingCentral Office account? SMS functionalty is free with an Office account.
Second, are you logging into your Android app with a digital line number? A toll-free number or any non-digital line number will not allow for SMS functionality.
Finally, in the most recent Android app, click on the carat symbol (
^ ) in the bottom menu. It will open up a sub-menu of additional functionality, including New Texts.