There are two different sites you can use to register. One is a TurboTax site and the other is an IRS site. Which one are you using? If one doesn't work, try the other. Can you either squeeze or expand the image on the screen on your phone to see the submit button?
https://turbotax.intuit.com/stimulus-check/
https://www.irs.gov/coronavirus/economic-impact-payments
